Hand held induction tool with energy delivery scheme

ABSTRACT

An apparatus and system for using magnetic fields to heat magnetically susceptible materials within and/or adjacent to adhesives, resins, or composites so as to reversibly or irreversibly bond, bind, or fasten opaque or non-opaque solid materials to one another. The system makes use of the effect that alternating magnetic fields induce eddy currents and generate heat within susceptors, and the effect that alternating magnetic fields additionally induce magnetic hysteresis that occurs in magnetic materials and thereby generate heat. An induction heating tool is used to emit the magnetic field at its work coil, and an electronic controller measures the energy being used by a power converter that generates the alternating current driving the work coil which creates the magnetic field. The distance between the susceptor and work coil is repeatedly analyzed based upon the power converter&#39;s input energy, and the work coil is driven at a repeatedly corrected power level during the heating cycle. Once a sufficient accumulated energy has been delivered to the susceptor, the magnetic field is turned off automatically by the tool, thus preventing overheating of the susceptor.

RELATED U.S. APPLICATION DATA

This is a divisional application of Ser. No. 09/705,282, filed on Nov.2, 2000, now U.S. Pat. No. 6,509,555.

This application claims the benefit of application No. 60/163,901 filedon Nov. 3, 1999.

TECHNICAL FIELD

The present invention relates generally to induction heating equipmentand is particularly directed to a hand held induction tool of the typewhich produces a magnetic field in a predetermined direction. Theinvention is specifically disclosed as an induction heating tool thatinduces eddy currents in thin foil susceptors to raise the temperatureof the susceptors to melt or soften an adhesive coating on thesusceptor's surface, for bonding trim to wallboard.

BACKGROUND OF THE INVENTION

Historically, bonding materials quickly without use of mechanicalfasteners has been quite difficult. Some of the problem areas whichexist include: assembly time, cost of materials and labor, quality ofthe bond between component structures being assembled, reliability ofthe process of assembly, the typical requirement of fairly difficultfinishing steps, convenience (or lack thereof) of use for the end-users,worker safety issues, and the difficulty in maintaining a good qualityof appearance of the finished goods.

In the 1940's, mechanical fasteners dominated the assembly industry, andadhesives were not as important to industry during this period. From the1930's through World War II, the United States and Germany began todevelop plastics and adhesives technology in response to the growingscarcity of natural products. In many cases, particularly early on,adhesives have been used either in combination with mechanical fastenersor where no mechanical fastener could be effectively employed. Beginningin the 1950's, the modern adhesives industry began to develop. Some oftoday's more common adhesive systems, developed at that time, includedheat-curable thermosets (epoxies), thermoplastic hot melts,pressure-sensitive adhesives (PSA's), contact cements, water-based woodglues, and the super glues (cyanoacrylates). These were major disruptivetechnologies that have evolved over the last 45 years and which haveslowly grown the fastening market and have significantly replacedtraditional mechanical fasteners in many markets.

Adhesive bonding is generally superior to mechanical fastening, butpresent technology doesn't allow for cost-effective pre-positioning andrapid development of a strong bond on demand with one step.Pre-positioning of components, prior to fastening, is very important,particularly in non-automated assembly systems. Millwork is an excellentexample of such an assembly system. No current adhesive system allowsfor pre-positioning coupled with instantaneous bonding. Most of today'sadhesives are slow to cure, requiring minutes to hours, thus requiringclamping and other direct personal attention by the installer.

It should be noted that the ideal adhesive system is one where theadhesive cures on demand, is reversible on demand, has unlimited shelflife, has no VOC's (volatile organic compounds), and is safe and easy tohandle. Currently, the only such systems that exist are thelight-curable systems, such as those employing UV (ultraviolet) andvisible light. UV and visible light systems are unique in today'sadhesives world. They are liquid systems that cure only upon exposure tolight. Optically transparent parts can be bonded in seconds or less tovirtually any substrate. Such systems, where useful, have virtuallyreplaced all other adhesive or mechanical fastening systems. An examplewould be automotive headlamp assemblies that do not need to bedisassembled. It should be noted that UV-curable adhesives involvechemical handling and are not currently reversible.

There are two basic types of adhesive systems: one group of systemsallows for pre-positioning of the parts to be bonded, yet by default,requires long cure times; the other group of systems provides veryshort, almost instantaneous cure times, but yet prevents pre-positioningof the parts.

Before describing some of the major adhesive systems available, oneshould be aware of the following general application notes that affectadhesive utility.

(1) Many product assembly sites are often dirty and difficult to keepclean. Certain adhesive systems cannot handle such situations.

(2) Temperature fluctuations at many assembly sites could be extreme,whether for an interior or exterior application. For example, a new homebeing built in the middle of the winter could see interior temperaturesbelow 0° F. Exterior applications could easily see temperatures evenlower. Another typical example could involve automotive body repair, ifdone inside a non-heated building.

(3) Where humidity may be important, it is clear that the humidityaround a manufacturing facility in Arizona would be far lower than thatin and around a facility in Florida.

(4) The ability to directly heat many product components to cureadhesives is extremely limited, particularly as many plastic componentscan melt, and wood-based or cellulose-based millwork can burn.

One family of fast-curing adhesives is called “super glues”(cyanoacrylates). These adhesives allow for an extremely rapid adhesivesetting, but cannot in any way be pre-positioned before placement as theadhesive cures during positioning. Thus, there is no room for error.These adhesives are generally the most expensive adhesives. Furthermore,they are difficult to handle, and have a limited shelf life. Finally,there is no way to easily reverse cyanoacrylate, or super glue, bonds.Companies in this industry include Loctite Corporation, Henkel A. G.,and National Starch.

Another instant adhesive technology, not often employed in structuralapplications, is pressure-sensitive adhesive (PSA) tapes. Like superglues, such products allow for extremely rapid adhesive bonding, butagain, are extremely limited with regard to pre-positioning and, as withthe “superglues,” again, there is no room for error. Furthermore,pressure-sensitive adhesives are limited in their ultimate strengthsunless they are thermosetting. In the case of a thermosetting PSA, someform of heat- or moisture-activation is required which is generallyimpractical for non-heat-resistant products, or where humidity controlsare unavailable.

The latter two above thermosetting processes are time intensive. Evenmore importantly, pressure sensitive adhesives can be applied only invery narrow temperature ranges, typically from 55° F. to 90° F.Furthermore, above 90° F., many common PSA's weaken dramatically. As afurther note on PSA's, they are incapable of flow without heat toaccommodate uneven surfaces, and if exposed to dust or otherparticulates, they immediately lose much of their potential adhesivestrength. Finally, it is extremely difficult, if not impossible in mostcases, to disassemble parts that use PSA's. Examples of companies thatmanufacture PSA's are 3M and Avery-Dennison, which are the two largestof the group. The cost of PSA's can range from being some of the mostinexpensive to some of the most expensive adhesives available today.

Hot melt adhesives are another example of an instantaneous or fast-curesystem that significantly limits the ability to pre-position parts. Suchadhesives are melted either in a large tank or in a small glue gun andare then dispensed as a molten material onto the parts. The parts arethen quickly mated, and the bond forms as the adhesive cools. Thecooling process can be as short as a few seconds to possibly as long asten or twenty seconds. As with the other instantaneous adhesives, thereis little room for error, particularly where a clean and thin bond lineis desired. Such limitations are the reasons that hot melt adhesives areused most extensively in the packaging industry and also for bondingsmall parts or surface areas. They are particularly useful in highlyautomated production systems, such as for sealing cereal boxes.Furthermore, such adhesives cannot be reheated after product assemblywithout significantly or entirely heating the product assembly.

On the positive side, hot melt adhesives are one-component, solid-state,zero VOC systems that have indefinite shelf life and, for the most part,are considered as plastics for regulatory and safety purposes.Furthermore, most hot melt adhesives are moderate to low in cost,especially when compared to the super glues or the light-curableadhesives. Examples of some leading hot melt manufacturers are Henkel A.G., Jowat, National Starch, H. B. Fuller, and Ato-Findley.

Other types of adhesive systems are those which are pre-positionable,but have long cure times. The most well known pre-positionable adhesivesare the epoxies. Epoxy adhesives generally have slow cure times, usuallyon the order of minutes to hours, or even days. Most epoxies aretwo-part systems that, when mixed, become activated and cure. Thecatalysts are in one or both parts and their concentrations determinehow quickly the epoxy adhesive will cure. In fact, if enough catalyst isadded, epoxies can become instantaneous systems that are notpre-positionable. Epoxies are not difficult to handle, but do requirespecial care as exposure can sometimes be detrimental to human health(causing skin irritations and burning).

Epoxies are among the strongest adhesives known, but require heat toachieve ultimate strength. A major problem with two-part epoxies is thatcure time can vary dramatically with temperature. In fact, some systemscure so rapidly at temperatures above 90° F. that they become almostunusable. At colder temperatures, e.g., below 60° F., some systems maytake days or more to cure. There also are one-component epoxies thatcure only upon exposure to heat. Once heated, many one-component systemscan cure in less than one minute. Epoxy bonds cannot be easily reversed.Examples of leading epoxy manufacturers include Ciba-Giegy, ShellChemical, Henkel A. G., and Loctite.

Urethanes are another well-known, pre-positionable adhesive group. Likethe epoxies, there are both two-part and one-part systems. Afterepoxies, urethanes are probably the second strongest class of commonlyused adhesives. Two-part systems are the most common and generally takeminutes to hours to cure. There are many one-part systems becomingavailable today which are moisture-curable (the moisture is actually asecond part). Both systems have the problem that one component of thetwo, the isocyanate, is moisture-sensitive. If water gets into theadhesive, or if the humidity is too high, the isocyanate will react withthe water, generate a gas, and cause foaming to occur. Even worse, ifthe moisture gets into a container unbeknownst to the user, and thecontainer is then closed, the container can explode. As a result,two-component and moisture-cure urethanes are generally only used byskilled or specially trained personnel. Furthermore, because of theirreactive nature and environmental susceptibility, most urethane adhesivesystems require specialized mixing and dispensing equipment that must becleaned on a frequent basis.

The primary advantage of most urethane adhesives is the availability ofroom temperature, moisture-curing, one-part systems that possess anoverall lower application viscosity. This is as opposed to a two-part,room temperature epoxy that must be mixed, or a one-component hot meltthat must be melted. Applications for urethane adhesives range fromautomotive assembly, to marine and aerospace assembly, to the millwork,furniture, and cabinetry industries.

It is important to note that certain adhesives have already been usedwith induction devices for many years. For example, such technologiesare used for high strength bonds using relatively long cure-time(fifteen minutes to hours) adhesives. Furthermore, this technologygenerally employs high pressures to facilitate bond formation. Thistechnology is used, for example, by Boeing, in the construction ofcomposite-based passenger aircraft. The adhesive systems employed byBoeing are mainly epoxies. Such adhesives must be pre-positionable, andfurther must be cured over a long period of time because of the strictperformance requirements mandated by the government for passenger andmilitary aircraft.

Another company that employs similar technology is Emabond, a subsidiaryof Ashland Chemical. Emabond develops the same types of long-time-cureadhesives (epoxies) as does Boeing, however, Emabond employs particulatesusceptors which activate at higher frequencies that require operatorshielding for safety. Emabond equipment is primarily geared towardautomotive component assembly. A special piece of induction equipment istypically required for any two automotive components to be assembled.

Emabond employs a number of adhesive technologies, including epoxies,urethanes, and hot melt adhesives. Most of the adhesive systems used byEmabond are heat-activated by particulate susceptors, not foilsusceptors, at higher frequencies that are known to be dangerous tohuman health (e.g., above 5 MHz). Moreover, the Emabond systems,primarily for the automotive industry, are part specific and aredesigned to bond generally irregular surfaces. The particulatesusceptors allow for the use of liquid adhesives that can easily conformto these irregularities.

One method of bonding structures together utilizes susceptors made of anelectrically conductive material that is heating by an alternatingmagnetic field to activate an adhesive material that resides on at leastone surface of the susceptor. The magnetic field induces electricalcurrents, known as eddy currents, in the electrically conductive media.Exposure of such electrically conductive media to a magnetic fieldcauses a temperature rise (heating) by what is termed the Joule effect.The Joule effect relates to heat generation due to the flow of electronsin a conductor. Distributions of these electrical currents and the heatthey produce are not uniform in a conductive medium, such as asusceptor, exposed to an alternating magnetic field. The magnitude ofheat, in Watts, is the sum of the heat contributions of all eddy currentpaths within the susceptor, each of which contributes heat that isequivalent to the product of its electrical resistance in Ohms and thesquare of its electron current in Amperes.

Within non-ferromagnetic susceptors, induced eddy currents have maximumintensities at the surfaces nearest the incident alternating magneticfield and have reduced intensities within the material, decreasingexponentially as a function of depth. This phenomenon is known as theskin effect, or the Kelvin effect, and the depth at which the eddycurrent falls to 37% is known as the depth of penetration. Mostsusceptors employed in the present application are comprised of a thinconductive sheet of uniform (or purposely non-uniform thickness) where,for the low frequencies usually used, the depth of penetration is fargreater than the material thickness. Eddy currents at all depths withinthese susceptors are thus approximately equal, except where purposefulvariations in susceptor thickness, or where open-space across the widthof the susceptors cause variations in current density. In such cases,currents are forced to be non-uniform in specific regions to create moreuniform heat generation or less uniform heat generation, depending onthe specific application.

The magnitude of heat generated within a susceptor comprised of aconductive sheet of uniform thickness, is related to several factors.These factors include susceptor permeability, resistivity, size andshape, and the magnitude, frequency, size, and shape of the incident ACmagnetic field. Variations of many of these parameters interrelate andaffect the current distributions and densities that affect the sizes andlocations of useful heat sources within the susceptors.

A Canadian patent by Krzeszowski, CA 1,110,961, (which is similar toU.S. Pat. No. 4,123,305) discloses a method for inductively heating athermo-fusible material interposed between a carpet and a floor. Aninductive heating tool is used to raise the temperature of a relativelythin-foil susceptor, which in turn activates the thermo-fusible adhesivematerial to create a bond, and thus “glue” the carpet to the floor.Krzeszowski discloses the use of a sheet of the thermo-fusible adhesivematerial, which is first placed upon the floor, followed by the carpet.Krzeszowski discloses the use of both continuously perforated sheets ofaluminum as the susceptor material, or solid aluminum sheet. In oneembodiment, a “vapour-barrier” sheet of aluminum (i.e., withoutperforations) is glued onto a slab of plaster, and then its other sideis glued to a slab of expanded polystyrene, thereby creating a moisturebarrier panel. One preferred aluminum sheet material disclosed inKrzeszowski is “ALBAL brand, reference 623,” either with or withoutperforations.

The Boeing Aircraft Company owns several patents in the field ofinductively heated susceptors. Virtually each patent extols the value of“even heating” of the susceptor to form a very high-strength and uniformbond. Of course, for aircraft structures, high strength bonds can becritical. Such patents include U.S. Pat. No. 3,996,402 (by Sindt), U.S.Pat. No. 5,717,191 (by Cristensen), U.S. Pat. No. 5,916,469 (by Scoles),and U.S. Pat. No. 5,500,511 (by Hansen). These patents use susceptorshaving various openings, and in some cases the openings are so large andnumerous that the susceptor has an appearance of a screen-like material.All of the susceptors specified by the above Boeing patents havethickness dimensions that exceed 0.003 inches (3 mils). Such devices arenot particularly useful in “quick” bonding of substrates.

Previous induction heating devices suffer from an inability to be madetruly portable, i.e., lightweight, while simultaneously delivering theenergy necessary to form bonds in short periods of time. It would bedesirable, especially for higher-speed, lower-strength bondingapplications, to provide an induction adhesive activation device withcorresponding susceptor design that accumulates the heat in thesusceptor and the adhesive while simultaneously withholding significantconduction losses to the substrates until all of the adhesive had eithermelted, begun chemical reaction, flowed adequately, or all threeoccurred.

Such a system would be valuable if the bonds developed were as strong astypically required for as wide a range of applications as possible, andit would be even more valuable if the susceptor adhesive device wereoptionally reversible by design. Such an induction adhesive activationdevice would ideally have improved energy efficiencies, sufficient toenable operation with a battery, be lightweight, support high duty-cycleoperation (>40%) for many hours at a time, and require no liquidcooling.

It would be advantageous to provide an induction-based adhesivetechnology that can bond nearly instantaneously on demand, and which isnot directed toward a pre-positionable adhesive, thereby allowing forsimplified, more rapid production, and eliminating the requirement ofhigh-energy systems such as those that operate at high frequencies thatare known to be dangerous to human health.

SUMMARY OF THE INVENTION

Accordingly, it is an advantage of the present invention to provide aninductive heating apparatus having a work coil that is driven by anelectrical current, in which an electrical current at a power converterstage is increased to a predetermined magnitude, whereupon the voltagemagnitude is measured and used to determine a “distance value” thatsubstantially represents the actual distance between the work coil and asusceptor, then based upon that distance value, the current and voltagemagnitudes are adjusted so as to cause said work coil to emit a magneticfield at a predetermined desired power level that will induce eddycurrents in the susceptor's electrical conductor to cause a temperaturerise.

It is another advantage of the present invention to provide an inductiveheating apparatus having a work coil that is driven by an electricalcurrent, in which both a current magnitude and a voltage magnitude of apower converter stage are sampled over multiple sampling time intervals,and the average current and voltage magnitudes are used to calculate apower level being generated, and the power level is then adjustedaccording to a profiled energy delivery scheme until achieving apredetermined accumulated energy for the profiled energy deliveryscheme, which ends the power cycle.

It is a further advantage of the present invention to provide asusceptor apparatus consisting of a strip-like structure having alength, width, and thickness, in which the structure includes at leastone layer of an electrically conductive material and at least one layerof an adhesive material proximal to at least one of its outer surfaces,wherein the electrically conductive material generates an eddy currentwhen exposed to a magnetic field of a predetermined minimum intensity,and the layer of electrically conductive material exhibits a thicknessin the range of 0.10 through 3 mils.

It is yet another advantage of the present invention to provide asusceptor apparatus consisting of a strip-like structure having alength, width, and thickness, in which the structure includes at leastone layer of an electrically conductive material and at least one layerof an adhesive material proximal to at least one of its outer surfaces,wherein the electrically conductive material generates an eddy currentwhen exposed to a magnetic field of a predetermined minimum intensity,and the at least one layer of an electrically conductive materialcomprises at least two individual layers of electrically conductivematerial, in which each of the electrically conductive layers isseparated from one another by an insulative layer.

It is still another advantage of the present invention to provide asusceptor apparatus consisting of a strip-like structure having alength, width, and thickness, in which the structure includes at leastone layer of an electrically conductive material and at least one layerof an adhesive material proximal to at least one of its outer surfaces,wherein the electrically conductive material generates an eddy currentwhen exposed to a magnetic field of a predetermined minimum intensity,and the structure contains at least one fusible portion that melts morequickly than other portions of said structure when its temperature israised.

Additional advantages and other novel features of the invention will beset forth in part in the description that follows and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned with the practice of the invention.

To achieve the foregoing and other advantages, and in accordance withone aspect of the present invention, a method for controlling energydelivered by a work coil comprises: providing a heating apparatus havinga work coil, and an electrical circuit that provides an electricalcurrent to the work coil; emitting a magnetic field from the work coil,increasing a first electrical characteristic of the electrical circuitto a predetermined first magnitude, and substantially at thepredetermined first magnitude, determining a second magnitude of asecond electrical characteristic of the electrical circuit to determinea distance value; and based substantially upon the distance value,adjusting the first electrical characteristic and the second electricalcharacteristic to emit the magnetic field at a predetermined desiredpower level useful at the distance value.

In accordance with another aspect of the present invention, a method forcontrolling energy dissipated in a susceptor being delivered by a workcoil comprises: providing a heating apparatus having a work coil, and anelectrical circuit including a power converter; emitting a magneticfield from the work coil, and directing the magnetic field toward asusceptor that contains an electrically conductive portion that produceseddy currents due to the magnetic field; adjusting a first electricalcharacteristic of the power converter to a first predeterminedmagnitude, determining a second electrical characteristic of the powerconverter substantially at the first predetermined magnitude, thendetermining a distance value substantially corresponding to a physicaldistance between the work coil and the susceptor; and based upon thedistance value, automatically adjusting the first and second electricalcharacteristics of the power converter to thereby emit the magneticfield at a power level sufficient to raise a temperature of theelectrically conductive portion of the susceptor.

In accordance with yet another aspect of the present invention, aheating apparatus is provided, comprising: an electronic circuitincluding a power converter stage, and a work coil, the electroniccircuit being configured to control a current magnitude and a voltagemagnitude at an input of the power converter stage; the electroniccircuit being configured to adjust one of the current magnitude orvoltage magnitude at the input of the power converter stage to a firstpredetermined magnitude, and to use the other magnitude to determine adistance value; and based upon the distance value, the electroniccircuit is further configured to adjust the current magnitude andvoltage magnitude at the input to the power converter stage so as tocause the work coil to emit a magnetic field at a predetermined desiredpower level.

In accordance with still another aspect of the present invention, aheating apparatus is provided, comprising: a work coil and an electroniccircuit, the electronic circuit, upon actuation of a cycle, beingconfigured to determine both a current magnitude and a voltage magnitudeinput to a power converter stage over a plurality of sampled timeintervals, then being configured to average the current and voltagemagnitudes to calculate a power level being consumed by the powerconverter stage; and the electronic circuit being further configured toadjust the power level according to a profiled energy delivery schemeuntil achieving a predetermined accumulated energy for the profiledenergy delivery scheme, and terminating the heating cycle.

In accordance with a further aspect of the present invention, asusceptor apparatus is provided, comprising: a substantially thinstructure having a length, width, and thickness, the structure includingat least one layer of an electrically conductive material, the structureincluding at least one layer of an adhesive material proximal to atleast one of its outer surfaces, and in which the at least one layer ofan electrically conductive material comprises a first layer ofelectrically conductive material, a second layer of electricallyconductive material, and a layer of electrically insulative materialpositioned therebetween; and the structure being of a characteristic bywhich the at least one layer of electrically conductive materialgenerates an eddy current when exposed to a magnetic field of apredetermined minimum intensity.

In accordance with yet a further aspect of the present invention, asusceptor apparatus is provided, comprising: a strip-like structurehaving a length, width, and thickness, and having at least one edge, thestructure including at least one layer of an electrically conductivematerial, the structure including at least one layer of an adhesivematerial proximal to at least one of its outer surfaces; and thestructure containing at least one fusible portion that melts morequickly than other portions of the structure when its temperature israised.

In accordance with still a further aspect of the present invention, anapparatus is provided having at least one structure made of a magneticmaterial having a predetermined shape to create at least two magneticpoles at times when a magnetic field flows therethrough; an electricallyconductive winding that is wrapped around at least one of the magneticpoles; and the structure is sub-divided into at least two portions, afirst of portion having a substantially curved end of a concave shape,and a second portion having a substantially curved end of a convex shapethat substantially mates against the concave curved end.

In accordance with still another aspect of the present invention, amethod of adhesive bonding by induction heating includes: providing asusceptor structure having a length, width, and thickness, the structureincluding at least one layer of an electrically conductive material andincluding at least one layer of an adhesive material proximal to atleast one of its outer surfaces, the structure being of a firstcharacteristic by which said at least one layer of electricallyconductive material generates an eddy current when exposed to a magneticfield of a predetermined minimum intensity, and the structure being of asecond characteristic by which the at least one layer of electricallyconductive material exhibits a thickness in the range of 0.01 mils (0.25microns) through 3 mils (76 microns); and exposing the susceptorstructure to a magnetic field during a heating event for a time intervalin the range of 0.05-10 seconds, inclusive, and at an average powerdensity in the range of 10-5000 Watts per square inch, inclusive.

Still other advantages of the present invention will become apparent tothose skilled in this art from the following description and drawingswherein there is described and shown a preferred embodiment of thisinvention in one of the best modes contemplated for carrying out theinvention. As will be realized, the invention is capable of otherdifferent embodiments, and its several details are capable ofmodification in various, obvious aspects all without departing from theinvention. Accordingly, the drawings and descriptions will be regardedas illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description and claims serve to explain the principlesof the invention. In the drawings:

FIG. 1 is a side elevational view of the outer case of a hand heldinduction heating tool, as constructed according to the principles ofthe present invention.

FIG. 2 is a side elevational view in partial cross-section of theinduction heating tool of FIG. 1.

FIGS. 3A-3B are an electrical schematic diagram of processing circuitand memory circuit elements of a first embodiment of the inductionheating tool of FIG. 1.

FIG. 4 is an electrical schematic diagram of a pair of timers used as analternative control circuit of the first embodiment of the presentinvention.

FIG. 5 is an electrical schematic diagram of an interface sub-assemblyof the first embodiment.

FIG. 6 is an electrical schematic of an inverter sub-assembly of thefirst embodiment.

FIG. 7 is an electrical schematic diagram of another invertersub-assembly which allows the induction heating tool to be powered by abattery.

FIG. 8A is a block diagram of the main electrical components of thefirst embodiment of the induction heating tool of the present invention.

FIG. 8B is a block diagram of the main electrical components of a secondembodiment of the induction heating tool of the present invention.

FIG. 9 is a diagrammatic view of current densities in an infinite sheetsusceptor used in the present invention.

FIG. 10 is a diagrammatic view of the current densities in a relativelysmaller-width susceptor used in the present invention.

FIG. 11 is a diagrammatic view of current densities in a susceptor thatis relatively smaller in width and includes holes or cut-outs, as usedin the present invention.

FIG. 12 is a diagrammatic view of the side of a work coil that emitsmagnetic field lines into a susceptor.

FIG. 13 is a top view of an infinite sheet susceptor illustratingapproximate relative forces on free electrons within a susceptorimmediately above, and axially centered on, a magnetic dipole.

FIGS. 14A-14B are an electrical schematic diagram showing the logic andmemory components, as well as the power and interfacing components, of asecond embodiment of the induction heating tool of the presentinvention.

FIG. 15 is an electrical schematic of a high frequency oscillator usedas an inverter in conjunction with the electrical schematic of FIGS.14A-14B.

FIG. 16 is a block diagram of an analog embodiment of the electronicsfor an induction heating tool as used in the present invention.

FIGS. 17A-17D are a flow chart of the major logical operations performedby the processing circuit of the second embodiment of the inductionheating tool of the present invention.

FIG. 18 is a perspective diagrammatic view of a double-foil susceptor,as according to the present invention.

FIG. 19 is a perspective diagrammatic view of a triple-foil susceptor,as according to the present invention.

FIG. 20 is a diagrammatic view of a susceptor having fusible lengths, asaccording to the present invention.

FIG. 21 is a diagrammatic view of a susceptor with fusible lengths and acurrent equalization hole pattern, as according to the presentinvention.

FIG. 22 is a side cut-away diagrammatic view of a susceptor used in thepresent invention after it has been bonded to two substrates.

FIG. 23 is a diagrammatic view of the induction heating tool of thepresent invention with a battery pack worn on a belt.

FIG. 24 is a diagrammatic view of the induction heating tool of thepresent invention with a battery pack worn on a shoulder harness.

FIG. 25 is a diagrammatic view of the induction heating tool of thepresent invention with a battery pack worn on a backpack.

FIG. 26 is a diagrammatic view of the induction heating tool of thepresent invention with a battery pack worn on a bandoleer-type shoulderharness, and further including an AC adapter or battery charger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the present preferred embodimentof the invention, an example of which is illustrated in the accompanyingdrawings, wherein, except for electrical component designations (e.g.,C1, R1), like numerals indicate the same elements throughout the views.

In a preferred embodiment, the present invention enables the use of hotmelt adhesive systems in millwork because the problem of “open time” iseliminated. Furthermore, reheating of adhesive and disassembly ofmillwork is possible when using the present invention. Virtually any hotmelt adhesive system currently in use today for millwork can be employedwith the present invention. This eliminates the need to develop new hotmelt adhesive systems for most millwork applications—particularlyinterior applications. A corollary to this is that there are manysuppliers from which to choose, and adhesive prices, even from a chosenstrategic supplier, should be competitive.

In an alternative susceptor/substrate structure of the presentinvention, a strip of susceptor material could be permanently bondedalong a surface of a (first) substrate material at the factory, and thenlater used for assembly on the construction job site to a (second)substrate material using the temperature-activated adhesive coating ofthe susceptor. In such a system, the susceptor would initially be bondedin the factory to the first substrate using, e.g., an epoxy or perhaps avery high temperature hot-melt adhesive. Of course, the idea is tochoose an adhesive that will not later become molten or softened whenthe induction heating tool is used on that susceptor.

On the job site, the (first) substrate material is brought into closephysical proximity to, or makes physical contact with, the (second)substrate, at the surface (or edge) where the susceptor has beenlocated. The induction heating tool is then used to bond thetemperature-activated adhesive on the susceptor's surface to the(second) substrate.

It should be noted that this alternative construction susceptor could beeasily disassembled by a new use of the induction heating tool. Oneadvantage in this methodology is that the susceptor would only de-bondat the surface of the second substrate upon this reverse assemblyprocedure—the first substrate and susceptor would not detach.

The present invention acts as a remotely usable induction heating tool,in which heat is developed in susceptors at distances of at least three(3) inches from the work coil, at preferred locations and withcontrolled spot sizes. Another aspect of the present invention is thatsusceptors can be made with sections having fusible links, the openingsof which are detectable by recognizing reduced power consumption. Thisallows an alternative embodiment induction heating tool to determine thesusceptor's temperature rate-of-rise, the “knowledge” of which enablesthe tool to automatically raise the remainder of the susceptor to anyarbitrary temperature, (preferably below its melting point),irrespective of the susceptor's distance from the tool.

In the present invention, predominantly non-magnetic susceptors areused. When such a susceptor is exposed to an alternating magnetic field,eddy currents are created within the susceptor, which in turn generate“repulsive” magnetic fields. Within a non-magnetic susceptor having athickness on the order of 10 mils or greater, the repulsive field sogenerated is strong enough to allow only a small fraction of theincident field from penetrating the susceptor. In such a case, theincident magnetic field is somewhat splayed, illustrated by magneticlines of force diverging much more quickly from the magnetic pole thatis sourcing this field, as compared to the profile of divergence withoutthis repulsive force (see FIG. 12).

Under these conditions, the magnitude of the incident magnetic fieldthat is left to penetrate a relatively thick susceptor is additive withthe magnitude of the repulsive field produced by the eddy currents,leaving a resultant magnetic field that produces little Joule heating.When thinner susceptors are used, however, their resistances are higherand the counteracting fields produced by their eddy currents arenecessarily smaller. As thinner and thinner foil susceptors are used, apoint is reached at which I²R, or the square of the current multipliedby the effective resistance, is a maximum, and the amount of Jouleheating reaches its peak value.

When using ever-thinner foils, the effective resistance of theelectrically conductive material can be plotted as rising linearly,inversely proportional to its thickness. The amount of Joule heatingthat is produced by these thinner foil susceptors, however, relatesnon-linearly to this increased resistance. The Joule heating rate isapproximately an exponential function of the foil susceptor thicknessas, synergistically, the opposing magnetic field generated by the eddycurrents decreases, and the resistance of the susceptor simultaneouslyrises in resistance. This fact is not addressed in the prior artliterature.

In some applications of the present invention, susceptors havingcontrolled widths are used in conjunction with work coils havingpredetermined ferrite diameters. A corresponding optimization ofgeometry produces what can be termed the “edge effect.” The effect ofcontrolled susceptor widths along with controlled foil thickness toconcentrate current flow on one side or both sides of, and along narrowsegments of, a susceptor, create controlled and efficiently heated areasof elevated temperature, or “hot spots.” The ability to concentrate suchheat development at a distance from a coil that otherwise imparts powerto a broad area within a conductive medium is a new and usefulapplication of projected induction heating. This ability is a secondsynergistic result of the crowding of eddy current paths into selectedcircular areas, and the crowding of current paths at one or both edgesof susceptors.

The above edge effect allows the induction heating tool of the presentinvention to heat a smaller overall area than is otherwise possible withnon-optimized components. This provides heating, and thus bonding, speedand efficiency, and allows the susceptor's heated areas along one orboth edges in an area to be just large enough to achieve the requiredbond strength. The induction heating tool irradiates an area of asusceptor that is sufficiently large to intercept the required magneticenergy, and yield eddy current concentrations that can yield speedy andefficient heating in a minimum area required for the particularapplication.

Within non-magnetic susceptors, maximum heating efficiency generallytakes place with uniform values of susceptor thickness in a preferredrange of 0.0001 inches (0.1 mils or 2.5 microns) to 0.003 inches (3 milsor 76 microns). The unique thickness at which this occurs for eachsusceptor is most particularly dependent upon the physical parametersof: (1) frequency of the incident magnetic field, (2) the geometry of,or most particularly the ratio of the susceptor width to the width ofthe ferrite core sourcing the incident magnetic field, (3) thesusceptor's material resistivity, and (4) the distance from the incidentmagnetic field.

Alternate susceptor materials include magnetic materials, where magneticpermeability is greater than 1.0. This permeability causes a naturalfocusing of the magnetic field since it causes densification of themagnetic field near the susceptor, as represented by a crowding ofmagnetic field lines of force. This helps to focus the magnetic energyof a work coil that is directed outward, with the associated advantagethat the repelling magnetic field and the consequent repulsion force donot exist. Similar optimization of these susceptors for the productionof localized heating is also readily achievable with the propermanipulation of susceptor width and thickness.

Materials to be bonded that are coated with susceptor material that isnon-uniform in shape make use of the possibility of minimizing the useof susceptive material and maximizing the effects of shapes that heatmost efficiently in various applications and work distances. In suchcases, the susceptive materials are placed at specific locations alongthe materials to be bonded, and to help identify these areas, a printedpeel-off coating can be attached to the outer surface where, afterapplication is completed, it can be detached and discarded. The peel-offcoating has the additional advantage of protecting finished surfacesfrom dirt, grease, and even disfigurement such as dents and scratches.In the absence of such printed peel-off coating identifiers, a variationin the tool that annunciates measured electrical parameters enables thetool to function as a susceptor-finder under low-power conditions sothat normal tool activation can be initiated at the proper locations.

It has been determined that, with respect to a variety of metals andalloys subjected to magnetic fields within the frequency range of about50 kHz to 150 kHz, that for non-magnetic susceptors maximum heatingefficiency generally takes place with uniform values of susceptorthickness within the range of 0.00001 inches (0.01 mils=0.25 microns) to0.002 inches (2 mils=51 microns), and for metals and alloys having aresistivity approximately equal to that of pure aluminum, maximumheating efficiency generally takes place more particularly within therange of 0.00002 inches (0.02 mils=0.51 microns) to 0.001 inches (1mil=25 microns), and most particularly within the range of 0.00005inches (0.05 mils=1.3 microns) to 0.0007 inches (0.7 mils=17.8 microns).

Results of testing have shown that, as frequency increases, the optimumsusceptor thickness generally decreases. Aluminum susceptors of varyingthickness have been tested, and the optimum thickness using a 130 kHz RFmagnetic field was found to be near 0.5 mils (13 microns). This optimumthickness is quite broad and operation at 0.3 mils (7.6 microns) doesnot result in a significant penalty. On the other hand, the optimumsusceptor thickness when using an 80 kHz magnetic field was found to begreater than 1.5 mils (38 microns), and at frequencies above 100 kHz,the optimum thickness was found to be between 0.3-0.5 mils (7.6-13microns).

The susceptor base resistance increases with frequency, as expected. Toincrease efficiency, the operating frequency could be selected to be ashigh as possible, limited by drive head and tank circuit losses.

An example of test results used a work coil constructed of a relativelylarge “U” core, wound with 8 turns. The tank circuit was operated at 220volts RMS, and delivered 59 W per pole to the susceptor. Thetemperatures were measured and the results recorded at 0.74 seconds heatcycle time using an optical pyrometer with a time constant of 5milliseconds. The energy deposited in the susceptor at that time was 44Joules. The temperatures did not exceed 250° C. and were above 100° C.over areas of about 2.5 cm² per pole.

Work coil optimization is similarly important to the efficiency ofenergy transfer at a distance. For example, a small U-core such as aMagnetics® OR44131-UC will not have the “reach” of a core that isspecifically optimized for a particular distance and susceptor geometry.As noted above, although only one magnetic field source, or pole, isinvolved in analysis of eddy current generation and Joule heating, aminimum of two such poles are always involved in the directing of theseincident fields. In one preferred case, for example, the core dimensionsthat resulted in the greatest energy transfer efficiency had dimensionsof about 1.75 inches in height (for a U-core), 3.25 inches width (acrossboth legs of the U-core), and a core area for the poles made of arectangular shape and having outer dimensions of about one (1) inch byone-half inch. The one inch dimension was in the direction of oneparticular susceptor axis.

This cross section needed to be about 3 cm² (½ in²) to keep losses to anacceptable level in the core, and the poles needed to be far enoughapart to limit interaction between the pole fluxes and maximize fluxlinking to the susceptor. With the inverter running at a maximum of 300volts RMS, the drive head needed to have 8 turns of the work coil todeliver 180W of power.

Calculations for designing the core in the above example, after defininga base set of component parameters, included the following:

1) The base 1-turn drive core inductance is Lo and the inductance with Nturns was:

L=Lo*N ²

Lo, was measured as:

Lo=82 nH

2) The base susceptor resistance was Ro and, with N turns on the core,it reflected to the primary winding as:

R=Ro*N ²

For the base susceptor, the measured resistance was:

Ro=500 μΩ

3) The base-drive current, Io, was determined from the susceptor powertransfer required and the susceptor resistance as:

Io=(P/Ro)^(1/2)

For the baseline case and a power transfer of 180 Watts:

Io=600 A-turn RMS

4) The base-drive tank capacitance was Co and for N turns was:

C=Co/N ²

Co was set from the operating frequency, ω, as:

Co=1/(ω² *Lo)

for the base case ω was 817000 rad/sec and:

Co=18 μF

5) The base tank operating voltage was Vo and for N turns was:

V=Vo*N

Vo was given by:

Vo=(Lo/Co)^(1/2) *Io

For the base case

Vo=40 volts RMS

The power supply operated at a maximum tank circuit voltage of about 300volts RMS, which required about eight (8) turns on the core (300 V/40 V)and it would have an inductance of about 5.2 μH (82 nH*64). Thecapacitance was to be about 280 nF (18 μF/64) for 130 kHz operation.Tank current was to be about 75 Amperes RMS. Power transfer to thesusceptor was about 180 Watts.

The core was to operate at about 0.15 T (1.5 kG). For Magnetics®“R”-type material at 130 kHz the losses were predicted to be about 310mW/cm³. The core volume was about 39 cm³, making the expected corelosses about 12 W. The Litz wire had a resistance of 5 mΩ/m. At 75Amperes RMS and one meter of wire, the wire losses were predicted to be28 W. Total drive head losses were predicted to be under 50 W, and theactual measured losses in the test unit were very close to 50 W.

An analysis of susceptor optimization was performed to understand how tomaximize power transferred to a susceptor. From very basic physicalprinciples, a relationship was developed between susceptor power and thekey problem parameters:

P∝σt(μ₀ωm₀ /y)²[1/(1+σtμ ₀ ωy)²]

Where:

P is the total power transferred to the susceptor,

σ is the electrical conductivity of the susceptor,

t is the thickness of the susceptor,

μ₀ is the permeability of free space,

ω is the frequency of the excitation,

m₀ is the magnetic moment of the tool pole, and

y is the separation between the pole and the susceptor.

From this relationship, it was expected that the expression, σtμ₀ωy, inthe second half of the equation defined a critical condition. Analysisproved that the critical value for this expression was about 10. Thatis: for σtμ₀ωy<<10, the power relationship reduced to:

P∝σt(μ₀ωm₀ /y)²

and for σtμ₀ωy>>10 it reduced to:

P∝(m ₀ /y)²[1/σt]

Further analysis revealed that for σtμ₀ωy≈10, an optimum susceptorheating condition was achieved. The optimum can be understood byinspecting the first equation and noting the dependence of susceptorpower on each parameter. In the above equation,

σ is the electrical conductivity of the susceptor. If electricalconductivity is zero then the power is zero. Power will increase asconductivity increases, but if conductivity becomes very large then thepower drops to zero again. This implies that there is an optimumconductivity that will maximize power transfer.

t is the thickness of the susceptor. The susceptor thickness displaysthe same dependencies as conductivity. Therefore, there is also anoptimum thickness that maximizes power transfer. In fact, by furtherinspection, it can be shown that the product of conductivity andthickness is the key susceptor parameter. For all practical purposes,two susceptors, one having half the conductivity of the other but twicethe thickness, will absorb exactly the same amount of energy from agiven induction field. In short, it will perform exactly the same.

μ₀ is the permeability of free space. Permeability is fixed in freespace. The effect of ferromagnetic susceptors requires a differentapproach and is not covered by this analysis.

ω is the frequency of the excitation. Power increases with increasingfrequency at low frequencies (i.e., when σtμ₀ωy<<10). At highfrequencies, the power becomes independent of frequency. There is,therefore, no optimum frequency that will maximize power transfer. Onewould always want to operate at the highest frequency possible,consistent with oscillator performance, component losses, andmagnetic-field-radiation safety.

m₀ is the magnetic moment of the tool pole. Power transfer increases asmoment squared.

y is the separation between the pole and the susceptor; power transferdecreases with increasing separation.

Further testing of aluminum susceptors revealed that an optimumthickness of aluminum for certain conditions was about 0.5 mils (12.5μm—microns), and that a generally optimum-thickness region is somewhatbroad with little variation in heating efficiency over a range from 8 μmto 20 μm. Power transfer was a maximum for a standard separationdistance of about ⅛ inch (3 mm) and for a 0.0005-inch (12.5 μm) aluminumsusceptor. Decreasing the susceptor thickness to 0.0003 inches (7.6 μm)resulted in a drop in power transfer of about 35%. Increasing thethickness to 0.0015 inches (38 μm) resulted in a drop in power transferof about 30%.

Susceptors made of brass were also tested, and these exhibited a maximumpower transfer at an optimum thickness of about 60 μm, almost exactly 5times greater than for aluminum. The product of conductivity andthickness is a key parameter, and for a given frequency and separation,any susceptor material could be optimized by simply selecting theappropriate thickness to optimize the product of thickness andconductivity. A higher conductivity brass may provide a greater powertransfer at a lower thickness.

For each operating frequency, there is an optimum susceptor. The optimumsusceptor thickness-conductivity product varies inversely withfrequency. The maximum power transfer, over a particular range ofdistance between work coil and susceptor, increases approximatelylinearly with frequency.

Another important relationship is the energy transfer efficiency as afunction of susceptor width. Direct correlation of power transfer isdifficult since narrow susceptors have less material to heat (susceptor,substrate and adhesive).

Susceptors made of steel were also tested, and the results for steelshow that enhanced power transfer could be achieved with its use.However, ferromagnetic materials behave differently fromnon-ferromagnetic materials. For the latter, the thickness of thesusceptor is very small compared to the electrical diffusion depth. Forexample in aluminum, the diffusion depth at 130 kHz is about 400 μm,almost 40 times the thickness of the baseline susceptor. For iron with apermeability of 2000, the diffusion depth at 130 kHz is about 20 μm,about the same as, or less than, the thickness of these susceptors.Therefore, one would expect ferromagnetic materials to be diffusionlimited, and power transfer could not be practically dependent onsusceptor thickness.

The susceptor selection and optimization process can be understood fromthe optimization relationship: σtμ₀ωy≈10. The susceptor optimizationparameter is the product, σt. The conductivity and thickness can alwaysbe selected to satisfy the optimization criteria and maximize powertransfer to the susceptor. The separation parameter, y, is driven by theapplication and not by a tool or susceptor design parameter. Theoperating frequency, ω, is a tool design parameter and is selected as atradeoff between susceptor power transfer and tool losses (size andcost). Ferro materials are diffusion limited and power transfer iscontrolled by a different mechanism.

The heating of the susceptor typically is non-uniform. There is intenseheating along the edges of the susceptor in the region of the polepiece, while the center may not be heated at all. Some test results showthat heating was not nearly as rapid as expected given the predictedpower transfer to the susceptor and the heat capacity of the susceptor.The concentration at the edge did not seem as pronounced as expected.The reason for these observed differences was thermal conduction, bothdirectly into the substrate (adhesive and wood) and laterally in thesusceptor.

Conduction into wood substrates was very important, since the edgetemperature of the aluminum susceptor, with no conduction to wood oneither side, would reach over 200° C. However, susceptors bonded to woodtake much longer to heat, and as a result, they do not become nearly ashot under the same magnetic field conditions and time intervals. Theenergy absorbed by the wood must come from the induction heating tool,and it contributes nothing directly to melting adhesive.

Testing showed that a 0.5 mil (13 micron) “standard” aluminum susceptoris very nearly optimum for a 0.75-inch (19 mm) separation and 130 kHzoperation. The resistance of the standard susceptor at 0.75 inches fromthe ferrite core of the work coil is about 110 μΩ per pole. Resultsusing an adhesive that melts or softens below 100° C. indicated that theheating system requires a power transfer to the susceptor of between 50W and 100 W per pole to achieve a good bond in 0.5 seconds. The use oflower conductivity materials would not reduce the power requirements;thicker susceptors would be required to reach optimum power transferconditions.

Other variations of work coils are useful for optimizing the creation ofeither the hot spots mentioned above, or uniformly heated areas of afoil susceptor. Although only one magnetic field source, or pole, isinvolved in the above discussions of eddy current generation and Jouleheating, a minimum of two such poles are always involved in thedirecting of these incident fields. The poles are generally set apart toeffect limited interaction between them, and this encourages thepreponderance of magnetic field lines, or components of magnetic fieldlines which have vectors that are more nearly parallel to the axes ofthe cores. In some cases, an increase in the ratio of components morenearly parallel to these axes, to the components that are more nearlyperpendicular to these axes (i.e., those more nearly parallel to thesusceptor)—where the efficiency of eddy current generation is at aminimum—can be accomplished by using ferrite cores that produce sucheffects, such as “E” or “U” cores that project the magnetic source thatis opposite in polarity to that in the center of the above-mentionedincident fields, outside the boundaries of the susceptors in particularapplications. This is done so that near the areas where the heatgeneration is to be maximized, the components of the magnetic field thatare more nearly parallel to the axis of the ferrite source are maximizedto produce eddy current maxima.

The induction heating tool of the present invention in its preferredembodiments is sufficiently small and of low weight to be portable, andcan be used on the job site as a hand-held device, either with a batterypower source or plugged into an AC line voltage outlet. The tool canautomatically deliver a prescribed amount of energy to ahot-melt-adhesive-coated susceptor to achieve a bond between twoobjects, at any distance up to at least 0.75 inches (19 mm), in lessthan one-half second, when using an adhesive that has a melt or softentemperature of below 100° C. In typical operation, it is programmed todeliver an energy level in the range of about 50-200 joules in this timeperiod over two susceptor areas, each about 2.5 cm². Adhesives that meltor soften between 100°-200° C. will typically bond within 1.5 seconds.

When using the preferred thin-foil susceptors of the present invention,the thickness of such aluminum susceptors (those in which the susceptorcontains aluminum as an alloy or layer) preferably is in the range(inclusive) of 0.01-2 mils (2.5-51 microns), or more preferably0.01-0.75 mils (0.25-19 microns), or most preferably 0.01-0.55 mils(0.25-14 microns). When using susceptors made of other electricallyconductive materials, the preferred thickness is in the range(inclusive) of 0.01-3 mils (0.25-76 microns), or more preferably 0.05-2mils (1.3-51 microns), or most preferably 0.01-1 mils (0.25-25 microns).

The above susceptors are heated by an alternating magnetic field, whichproduces a power density in the susceptors, and is applied forrelatively short time intervals. The preferred values are as follows:

the heating cycle time duration is in the range of 0.1-10 seconds(inclusive), or more preferably 0.1-5 seconds (inclusive), or mostpreferably 0.1-2 seconds (inclusive);

the power density is in the range of 10-5000 Watts per square inch ofsusceptor (inclusive), or more preferably less than 1000 Watts persquare inch, or most preferably less than 500 Watts per square inch; and

the operating frequency of the alternating magnetic field is in therange of 1 kHz—1 MHz (inclusive), or more preferably 10-500 kHz(inclusive), or most preferably 10-300 kHz (inclusive).

The “heat” energy is supplied by the work coil (an electrical inductor)in the form of a magnetic field. The work coil is literally anelectrically conductive wire wound onto two identical ferrite posts thatbecome the legs of a three-piece U-core. In the preferred embodiments,the work coil wiring is made up of Litz wire, which has very littlereactive losses due to skin effect at the radio frequencies of operationof the tool.

The two flat ends of the ferrite core emit the AC magnetic field. Whenthis field is presented to an electrically conductive material (i.e.,the susceptor), free electrons within the material are set in motion.This electron motion within the conductive medium tends to mirror theelectron motion in the coil. This flow of current through thesusceptor's electrical resistance produces a dissipation of power thatresults in the generation of heat.

When the temperature of the susceptor rises it causes the adhesivecoating to melt, or at least soften, and the adhesive flows on thesusceptor surfaces. At the end of the heating cycle, the adhesiverapidly cools due to heat loss into deeper and cooler levels of thematerials being bonded (called the substrates), and a strong, permanentbond is produced that will remain strong unless the susceptor is laterreheated with a similar induction tool.

The energy that creates the magnetic field is derived from avariable-output power oscillator circuit (typically referred to as aninverter) operating at a frequency of about 130 kHz in the preferredembodiment. Electrical power for this inverter is supplied by avariable-voltage power converter which is controlled by amicroprocessor. Power delivered to the susceptor is sensed by amicroprocessor through active monitoring of the power converter's inputvoltage and current, in a “feed-forward” control configuration. Byinitiating each heating cycle at low power and quickly ramping theapplied voltage to achieve a particular current magnitude that willdeliver a programmable number of joules in about one-half second, thecontrol circuit automatically adjusts its output power to achieveconstant energy delivery.

In a second preferred embodiment, such adjustments can occur at a rateof about 11,000 each second, thereby easily compensating for inputvoltage variations, including much of the AC ripple on the DC powerrails. A look-up table is used in a second preferred embodiment tocompensate for variations in circuit power losses, thus allowing thetool to accurately operate over the entire operating distance range. Thetool can be programmed in the field using a laptop computer to deliverdifferent amounts of energy in different time periods to meet otherapplication requirements (such as those with different types ofsusceptors or substrates, or ambient temperature variations), and forresearch purposes. The tool can automatically control the power level inrepetitive firing cycles to maintain a substantially constant rate ofenergy dissipation in the susceptor itself during the overall timeinterval while the tool emits a magnetic field, even if the distancebetween the susceptor and work coil changes during the overall timeinterval.

The control circuit monitors the work coil temperature and activates afan when the work coil's temperature exceeds a programmable level. Italso operates four high-output LED's that are used for illumination ofthe work surface, extinguishing these whenever the tool has been dormantfor more than a programmable time interval, e.g., thirty (30) seconds.Further, it provides tactile feedback to the user by initiating anoticeable vibration when the heating cycle is complete. It also storesinformation on thousands of activations, including activation timeperiod, calculated energy-output level, time since previous activation,and work coil temperature.

As noted above, Litz wire is used to lower the work coil's effectiveresistance to allow the work coil to run at a relatively lowtemperature. Litz wire is composed of numerous individually insulatedstrands of wire and offers increased effective cross-sectional area dueto the “skin effect” which relates to the tendency for higher-frequencycurrents to flow on outer conductor surfaces. Since energy transfer issomewhat proportional to frequency, the operating frequency is balancedagainst core losses to determine the frequency that yields the optimumefficiency for the components used. In some bonding applications,susceptor dimensions are also controlled to maximize the tool'sefficiency (as discussed above). The optimized susceptor allows theinventive system to heat, at a distance, a smaller overall area than isotherwise possible with non-optimized components. This allowsachievement of speed and efficiency of heating and bonding.

The present invention offers many advantages, such as reduced assemblytimes, which in the past typically required hours, but which now can bedone in minutes or seconds. Other advantages include: elimination ofadhesive open-time constraints; elimination of hot melt safety issues;elimination of dangerous VOC's; elimination of need for surfacerefinishing; reduced assembly costs; improved bond quality; improvedprocess reliability; improved process convenience; bond reversibilitywhere desired; and allowance for the use of adhesives to bond opaquematerials where quick set time and pre-positioning of the material ishighly desirable.

The adhesive products used with the susceptors that are available foruse with the heating induction tool of the present invention add valueto assemblies through improved appearance and reliability for the enduser, typically at a reduced cost to, and at a greater convenience for,the assembler, installer, or manufacturer, by: enabling the use ofvirtually any material; enabling the use of low-cost substrates withhigh quality finishes; eliminating the need for mechanical fasteners;allowing for the pre-finishing of virtually any material; andeliminating the need for post finishing.

The present invention improves overall product quality and reliabilityin the field of bonding as compared to that offered by mechanicalfasteners, particularly where adhesives are not used today, by:eliminating the use of mechanical fasteners; increasing specific bond orconnection strength between two components; allowing for the use of anymaterial other than metal (although metal can be used in someapplications); and reducing product manufacturing costs.

Manufacturing costs can be reduced by: eliminating or acceleratingfinishing steps, eliminating or accelerating assembly steps, eliminatingthe need for an adhesive with a mechanical fastener, significantlyaccelerating adhesive set time, allowing for reversal of productassembly (optionally where damage of the parts needs to be minimized),allowing for more economical materials selection, allowing for moreeconomical adhesive use, and providing more compact equipment that takesup less floor space, requires lower power, and requires less maintenancethan large-scale production equipment.

Manufacturing costs can also be reduced by: allowing for thepre-application of adhesives, which avoids the need to comply withstrict federal and state regulatory requirements, allowing for theelimination of VOC-emitting adhesives, eliminating chemical handling andodors at the end user's site by pre-applying the adhesive, and reducingoverall process costs.

Referring now to the drawings, FIG. 1 shows a hand held inductionheating tool, generally designated by the reference numeral 10. Theinduction heating tool 10 consists of certain major portions, includinga handle portion 20, a bottom case portion 30, a top case portion 60,and a work coil “head” portion 50.

The handle portion 20 includes a finger-operated trigger at 22. Thebottom portion 30 includes a power cord fixture to receive a power cord32, that typically would plug into a standard 120 volt AC outlet. Thetop portion 60 includes some LED's 34, used for providing warning orstatus indications to the user/operator of the tool 10. The top portion60 also includes some LED's at 64 to illuminate the work piece, ifnecessary. Additional lighting LED's 64 are included in the bottomportion 30, which can be viewed in FIG. 2.

In some configurations of the present invention, the work coil “head” 50is interchangeable with other heads of various sizes and output powerratings, for the same, or other induction heating applications (such asthe heating of large susceptors used to bond laminates and other sheetgoods).

Referring now to FIG. 2, the same type of hand held induction heatingtool 10 is again illustrated, this time in partial cross-section so asto be able to view some of the major components that are containedwithin the case. For example, the trigger 22 is mechanically incommunication with a trigger switch 24. As can be seen when comparingFIGS. 1 and 2, the trigger structure 22 can be of various shapes andsizes.

FIG. 2 also illustrates a feedback solenoid 26 that is contained withinthe handle portion 20, although in some embodiments a “buzzer” motorthat drives an off-center cam buzzer device, or other vibrator device,is utilized in lieu of the feedback solenoid 26.

The rear handle portion 20 also includes a printed circuit board at 40,which includes the voltage regulator sections and the processing andcontrol circuitry, examples of which are found schematically in FIGS.3A-3B and 14A-14B. The regulation and control circuitry could becombined with power components, if desired, but in the illustratedembodiment of FIG. 2, an inverter printed circuit board sub-assembly at62 is illustrated as a separate component that is contained within thetop portion 60.

The bottom handle portion 30 also includes a communications port 36 thatcorresponds roughly to the communications port 116 on FIG. 3, orcorresponds to a connector for an infrared communications port 560 onFIG. 14.

A power supply is included to provide 12 volts DC, and adjacent to this12 VDC power supply is a printed circuit board that contains severalparallel capacitors at 44. These capacitors 44 are in close proximity tothe work coil sub-assembly 50, and the work coil itself is a powerinductor made up of an U-core at 54, preferably made of ferrite, andcontaining multiple windings of an electrical conductor at 52, such asLitz wire or other copper wire. The work coil and capacitors at 44create a resonant circuit that will oscillate at a predeterminedfrequency, when energized.

The work coil head 50 includes ferrite core pieces in one of thepreferred embodiments, as noted above. In one construction of thisferrite material, the core 54 is sub-divided into three separate piecesalong the curved lines 58 on FIG. 2. This construction allows the workcoil to undergo additional vibration and shock mechanical loads with alesser chance of breakage in the relatively brittle ferrite pieces, byallowing the three separate ferrite core pieces to “pivot” along thesecurved lines 58. The magnetic circuit remains intact, since there is nopurposeful air gap in core 54 at these curved lines 58.

During prolonged use, the work coil area may get quite warm, andtherefore, a fan 56 is provided to lower the temperature in the workcoil area (i.e., within the interchangeable work coil head 50).

The induction heating tool of the present invention is designed toaccept a variety of work coils, each of which is used for a specificapplication. Each work coil contains resonating capacitors so that themajority of the coil current will not flow through connectors.

The power supply of another alternative embodiment induction heatingtool uses a “swinging choke” that varies its inductance value as coilcurrent is varied. The coil, or choke, diminishes in inductance, andtherefore, energy storage capacity, as current drawn by the susceptor isincreased. This property enables the switching transistors to operate ata more constant voltage level as the current increases, rather thanexperiencing a rise in peak voltage as current is increased, therebypreventing excessive voltage levels to be experienced by thesetransistors. The means of achieving this swinging choke characteristicis to create the usual gap in the core that is required to causesaturation at lower levels of magnetic flux.

Extra fine Litz wire is used in the work coil, which produces less heatloss in the electrical conductors of the coil. Ferrite materialcomprises the cores of the work coils. Curved ends of this ferritematerial (at reference numeral 58) act as joints against whichconnecting (sometimes brittle) ferrite pieces or sections can movewithout transmitting high torsion and tensile forces to minimizepotential breakage, while still maintaining good magnetic coupling.

Some of the other important aspects of the induction heating tool of thepresent invention include: the tool is portable; it can be batteryoperated; the tool's energy delivery within the operating range of thetool can be kept nearly constant; the tool will shut down afteridentifying programmable (adjustable limit) error conditions such asover-temperature, under-power, and over-power (the latter two bydiscriminating between an “in-range” susceptor and an “out-of-range”susceptor, including no susceptor, and one that draws too much power);the tool can be programmed to deliver a profiled power curve to controlthe temperature rise of a susceptor; multiple output energy levels canbe programmed; the tool can operate in a “continuous” (re-triggered)mode and automatically deliver energy bursts after adjustable delaytimes; and operating data can be stored for later analysis including,for each energy delivery burst, time since last energy burst, work coiltemperature, peak current, maximum and minimum voltage, and errorcondition(s), if any; annunciation of operating modes, levels and errorconditions is easily interpreted from a multiple-segment bar graphdisplay and/or from downloaded data.

In one mode of operating the induction heating tool of the presentinvention, the power curve is profiled at a rate that substantiallyincreases the power output near the beginning of a heating event whenthe tool is operated in conditions of low ambient temperature. Asdiscussed above, temperature fluctuations at many assembly sites couldbe extreme, especially in cold conditions in more-northern latitudes.The profiled power output could overcome some of the effects of the coldon the adhesive of the susceptor by providing “extra” power near or atthe beginning of the power ON heating event to begin raising thesusceptor's temperature more quickly, as compared to the power profilethat would otherwise be used in more moderate conditions. Of course, theoverall accumulated energy delivered must be also considered, and it maybe that the increased power output levels should remain somewhat“increased” throughout the entire heating event in really coldconditions.

Similarly, in conditions of relatively high ambient temperatures, theprofiled power curve could be modified to substantially decrease thepower output.

In addition to the above, the induction heating tool of the presentinvention will automatically shut down under several conditions,including: an under-power condition, when confronting an “air load;” anover-temperature condition at the work coil; a “timed out” condition,where the load power is too low, but greater than the “air load” powercondition; or when the programmed energy level is reached. In one modeof a preferred embodiment, the delay to shutdown upon an “air load”condition is 75 msec, and is not user settable; the “timed out” shutdowncondition is programmable by the user in one msec steps; and theover-temperature limit is programmable in degrees C.

In the first preferred embodiment, a microprocessor 114 is utilized inthe electronic circuit of the induction heating tool. One exemplaryintegrated circuit microprocessor is a Philips 80C550 microprocessor,which contains 4K of onboard ROM and 128 bytes of RAM. To maintainvariable values, including user settings, in a non-volatile memory, anEEPROM chip is provided. Operating parameter storage is contained in a2K×1 serial EEPROM, such as the bank of EEPROM's at 140 on FIGS. 3A-3B.Such stored operating parameters include maximum tool power output,maximum voltage and current values, low-load and overload current andpower levels, multiple operating power levels, current power level, andother common parameters, as required.

Referring now to FIG. 3 (FIGS. 3A-3B), an electrical schematic diagram100 depicts the controller component of a first preferred embodiment ofthe induction heating tool 10. In this exemplary circuit, a Philipsmicroprocessor 114 (U9) is used, part number 80C550. Several serialEEPROM memory chips (U1-U8) are provided at 140, which store operatingdata, if desired, and which store operating parameters or limits, someof which can be user-settable. Other interfacing circuit components areprovided, such as a crystal oscillator circuit 112 (Y1, C17, C18) and asmall filter capacitor C9 across the +5 VDC power rail.

It will be understood that the microprocessor 114 could be provided withon-board memory, such as EPROM or EEPROM to store operating parametersand perhaps some accumulated data. In such a microprocessor, the serialEEPROM memory chips at 140 could be eliminated in models of the heatingtool that do not require historical operating data to be stored andlater downloaded to a host computer.

Other filter capacitors across the +5V rail are used in variouslocations on the printed circuit board 40 that contains this circuit(e.g., C1-C8, C13, C16, C23). The +5V rail is signal-conditioned by aninductor L1 and capacitors C26-C27.

An RS-232 level converter circuit 118 is provided (U12, C19-C20,C21-C22, R3-R4) which allows the microprocessor 114 to communicate witha host computer (not shown on FIG. 3). In addition, an external watchdogtimer circuit at 110 (U10, U11, Q1, Q3, C10, C24-C25, R11-R12, R16-R18),and a status indicator LED array 130 (LED0-LED7) are provided. A powerON LED indicator at J14 is also provided.

The circuit on diagram 100 includes digital inputs and outputs, such asthe RS-232 input data stream and output data stream (at the R×D and T×Dpinouts on U9 and U12), and a trigger switch signal 134 through R2(along with R6) from the operator control switch (e.g., switch 24 onFIG. 2). One of the other digital outputs include a signal that controlsa signaling device (e.g., a buzzer or solenoid) to alert theuser/operator of the induction heating tool 10 that the end of theheating cycle has been reached. The interfacing components for thisfunction are the circuit 120, including R7, R13, Q2, and D1. The buzzeroutput drive signal is at 122.

The multiple LED's of the LED array 130 are also driven by furtherdigital outputs of the microprocessor 114. These LED's are physicallygrouped as a bar graph display.

There are also certain analog inputs at 116 on diagram 100, includinginverter voltage, inverter current, and work coil temperature, usingR8-R10, C11-C12, and C15. A power adjust analog input signal is alsoprovided, using R19 and C14. An analog output is provided as a PWMproportional signal 132 (using R1).

The controller circuit of diagram 100 provides two modes of operation,in which the first mode is a “programming and data download mode.” Thetool 10 is connected via the RS-232 interface 118 to a host computerthat can upload operating parameters to the tool, and download thecontents of the EEPROM memory circuit 140 from the tool. In this mode,the tool 10 can be powered through the RS-232 cable.

In the second operating mode, called “normal,” upon a power-up thesoftware program first reads in operating parameters from the EEPROMmemory circuit 140. If the data read has an error, a set of hardcodeddefault values is used. The processor registers and memory are alsoinitialized.

A main operating loop is now entered, in which the tool continuouslymonitors the analog inputs and maintains a running average of invertervoltage and current. The work coil temperature is also monitored, and anerror condition is declared if any analog input value goes outside ofpredetermined limits.

When the trigger switch 24 is actuated, the tool 10 begins a “susceptorfind” algorithm, and the inverter 250 is energized with short currentpulses, and the inverter current is monitored. When the tool 10 isbrought within working distance of a susceptor, the inverter currentwill rise, indicating that the tool is properly oriented to make a bond.The LED indicator 130 is flashed to alert the user/operator of thiscondition.

A heating cycle now commences, in which the inverter is turned on at apredetermined level, and its input voltage and current are multipliedmathematically to give input power. The power drawn is integrated overtime to provide a running measure of energy used by the inverter. Thecontrol software corrects this value for inverter and work coil losses,and compares the corrected energy level to a predetermined limit. Whenthis limit of total energy is reached, the inverter is shut off and theLED indicator and a buzzer or other tactile signaling device are enableto alert the user/operator of a successful bond.

The tool 10 can be operated in a repetitive mode, in which the heatcycle is repeated with a programmable delay between “heat shots.” Thisallows the user/operator to move the tool 10 along a trim item and makenumerous joints or bonds without being required to pull the trigger oncefor each joint/bond.

The background (watchdog) timer 110 is started at the beginning of eachbonding cycle. If this timer 110 should time out before the requiredenergy is accumulated in the susceptor, an error signal is generated andthe LED display 130 indicates this condition. Other error conditionsmonitored by this controller circuit 100 include inverter over-currentand work coil over-temperature. The LED display 130 can provide a uniquepattern for each of the error conditions.

At the end of each heating cycle, the peak inverter voltage, current,and work coil temperature can be saved in the EEPROM memory circuit 140,as well as the “shot number” and the time interval since the last shot.This data is available to be downloaded to the host computer (asdiscussed above) in order to monitor tool usage and operatingconditions.

In the first preferred embodiment, as noted above, tool operatingparameters are accessible through a serial port connection (at 116 onFIGS. 3A-3B) to a PC (not shown on FIG. 3). A custom computer programrunning on the PC provides supervisory control and data recoveryfunctions. Data recording can be stored in, for example, seven of the256K×1 serial EEPROM's (at 140), which allow up to 224 Kbytes ofnon-volatile data storage. Possible contents of the data record beingstored in the non-volatile memory include: time since last applicationof the tool (using, e.g., a 10 msec resolution); work coil temperature;peak current; minimum and maximum voltage; and error conditions, if any.

In the first preferred embodiment, two modes of operation are supportedas noted above: continuous (re-triggered) and one-shot. Each mode canselect one of several preset delivery schemes, each of which can beeither a maximum energy delivery or a profiled energy delivery.

The continuous or re-triggered mode will repeatedly self-trigger anenergy delivery cycle for as long as the trigger is pulled. Overload andlow-load error conditions will not terminate this cycle unless allowedto persist for a preset number of sampled time intervals. In the case oferrors, the inverter 250 (see FIG. 6) will be pulsed on at minimum dutycycle until a timeout occurs or the tool senses a proper load condition,in which case it will continue as normal. The cycle terminates instantlyon trigger release. A parameter is included to determine the dead timebetween cycles.

The single shot mode performs exactly one power cycle per trigger pull.Any error condition will immediately terminate the cycle.

The maximum-energy mode dictates the power output level of the inductionheating tool (in a range of 10% to 100%) and the target energy to besupplied to the load (e.g., a susceptor). A maximum powered-output timeis also provided. When the tool is triggered, it will sample bothcurrent and voltage during the ON time of the inverter, average thesevalues over multiple (e.g., eight) consecutive readings, and calculatethe power delivered to the load (susceptor). If this power is less thana low-load limit, then a low-load error condition is registered, andoperation is suspended until the trigger is released and again pulled.If the power or current levels exceed safe limits, then an overloaderror will be registered, and again the tool will be disabled untilagain triggered.

Once the maximum energy level is reached or exceeded, the cycle is endedas discussed above. If the maximum time is reached before this maximumenergy level is reached, a timeout error is posted and operation ishalted. A new cycle is automatically started after timeout in thecontinuous operation mode.

Profiled energy delivery uses a table of multiple (e.g., up to ten)steps. Each step is defined as power to be delivered during successive50-millisecond (or other time interval) steps, and power level isdefined in percent of full power. Low-load and overload conditions applyas described above. During the profiled power delivery, the tool willdeliver power as per the step's power value for the time specified, thenadvance to the next step. A maximum energy level can also be provided ina maximum energy mode; if this parameter is set to zero, no tracking ofenergy delivered will be performed. The cycle is terminated when theentire power profile has been performed. If this value is non-zero, theenergy delivery will be terminated if this value is exceeded.

In either power delivery scheme, inverter output power is delivered in50-msec (or other time interval) minimum steps. A 50% duty cycle couldbe, for example, 50 msec ON, followed by 50 msec OFF. Resolution ofpower duty cycle can be any predetermined time interval, such as 1 msec.In a preferred mode, the actual power level is determined by the OFFtime—for example, 50 msec ON and 1 msec OFF would be 50/51%, or a 98%duty cycle. 100% duty cycle is obtained by beginning the next 50 msec ONperiod immediately following the end of the present period.

In one mode of the first preferred embodiment, all error conditionsregistered are saved in the data memory (i.e., EEPROM's 140), and linkedto a data record in which they occurred. Distinct codes for the variouserrors are used; for example, one code each for overload, no load,timeout, over-temperature, and other error conditions. When an errorcondition is registered, an on-board LED bar graph (see 212 on FIG. 5)will flash all of its eight segments at a two (2) Hertz rate, while thetrigger 22 remains depressed. The error indication ceases on triggerrelease, and generally will not prevent subsequent operation.

An exceppion to the above statement is a work coil over-temperaturecondition. In this case, a distinctive LED pattern is displayed on theon-board LED bar graph 212; a “chasing” LED that travels from the lowestbar to the highest, and repeats by returning to the lowest bar, using atwo (2) Hertz stepping rate. This type of display will continue untilthe work coil temperature falls to a safe level. Operation of theheating tool 10 will be disabled during over-temperature conditions.

In normal operation, the bar graph 212 has no indication until thetrigger 22 is pulled. The operating mode chosen (potentially one ofeight possibilities in a preferred mode) will be displayed as thecorresponding LED “bar” illuminated. In the one-shot mode, thisindication remains during the operating cycle, then extinguishes at theend of the cycle to indicate that the cycle is complete. In continuousmode, the LED bar 212 will flash on at the beginning of each cycle andextinguish at the end of each cycle.

An alternative scheme that can be incorporated in the heating tool 10 isone in which the bar graph registers the operating mode (potentially oneof eight, as above) during non-triggered periods, and instead registersactual peak output power while the unit is operating. In thiscircumstance, peak operating power would be indicated on the “bargraph,” with all appropriate bar graph elements simultaneouslyilluminated.

Referring now to FIG. 4, an electrical schematic diagram 150 shows adual 555-type timer chip that is provided at 152 to replace thecontroller of FIGS. 3A-3B, in an alternative embodiment. This integratedcircuit device (called a “556” chip) contains two separate timers 160and 162, and on FIG. 4 the first timer is illustrated along theleft-hand side of the pin-outs (i.e., pins 1-6), while the second timeris illustrated along the right-hand side pin-outs (i.e., pins 8-13).Pins 7 and 14 are for the power supply rail and DC common.

On the first timer side, the timing elements include an adjustableresistor (or potentiometer) R1, which is connected to a 13 volt Zenerdiode D1. Connected to the wiper of potentiometer R1 is a fixed resistorR2 which is connected to the discharge terminal at pin 1. This is alsoconnected to the threshold terminal at pin 2, which in turn is alsoconnected to a capacitor C2. The potentiometer R1 is used to vary thetime duration of the first timer 160, and hence the energy output of thework coil of the heating induction tool 10.

This first timer 160 also has a reset pin, a trigger pin, and an outputpin. Resistor R7 and capacitor C4, along with capacitor chargingresistors R6 and R12, make up the timing elements that act as a simple“one-shot” circuit that starts this first timer 160 when the triggerswitch 24 is activated. This trigger switch on the schematic diagram ofFIG. 4 corresponds to the trigger switch 22 illustrated in FIGS. 1 and2.

The trigger input of the second timer 162 is fed from the output of thefirst timer 160, through a resistor R9 and a capacitor C6 (comprising asecond simple one-shot in which R8 is a pull-up resistor). This beginsthe operation of the tool-inhibit timer 162, which acts to disable timer160 for a preset time interval, to limit the operational duty cycle ofthe tool. This inhibit signal from timer 162 is fed to timer 160 throughdiode D2. With regard to the second timer 162, its threshold is set bythe +13 VDC rail and through a resistor R4, which has a small timingcapacitor C3.

The output pin of the second timer 162 connects to a third simpleone-shot formed by a capacitor C5 and resistor R11 to drive through R10,into the gate of a MOSFET transistor Q1. This power transistor Q1actuates a solenoid 166, and operates with a diode D3 to act as a driver164 for the solenoid. The solenoid 166 and MOSFET driver 164 switch raw160 volts DC, that is provided as a medium-voltage DC power rail at 154.This 160 VDC power rail 154 drives through a resistor R3, with filteringprovided by a capacitor C1 to provide the 13 volt DC power railcontrolled by the Zener diode D1.

The output signal of the second timer 162, as discussed above, creates aminimum OFF time between the stop of one cycle of operation of the workcoil and the start of the next operation cycle of the work coil. Thisprevents a “quick” re-triggering action that otherwise might cause thework coil to rise in temperature too quickly. The output of the secondtimer 162 also provides pulses to trigger the solenoid 166.

Referring now to FIG. 5, an electrical schematic diagram 200 is providedrepresenting an interface sub-assembly. A 120 VAC input is utilized, andis connected to a power input stage 202. This power input stage 202includes a step-down transformer, a bridge rectifier, a filtercapacitor, and a voltage regulator 204. The output of this voltageregulator 204 is a +5 volt DC rail.

A microprocessor sub-assembly 210 (which corresponds to FIGS. 3A-3B) isprovided that includes a multi-segment bar graph 212 that generallycomprises a series of LED's. Microprocessor sub-assembly 210 alsoincludes at least two A/D converters (analog-to-digital converters) thatare used to detect levels of DC voltage and current. One of these inputsat 214 represents the input voltage at the power oscillator (inverter250), which is signal-conditioned by some resistors and a filtercapacitor before arriving at one of the A/D converter inputs within themicroprocessor sub-assembly 210.

The load current (of inverter 250) travels through a portion 220 of thisinterface sub-assembly circuit, starting at a point 230, as indicated onFIG. 5. This load current travels through an FET transistor at 232, andthen through two “sense” resistors 234 and 236. The “sense” voltage atthe point 238 on FIG. 5 is sent through a 2.4V Zener diode to the gateof a 2N5060 SCR which can latch ON in an overcurrent condition,providing a turn-on of the 2N3904 transistor and a subsequent turn-offof the FET transistor at 232, thereby causing the tool to shut down.

The “sense” voltage 238 is provided to an amplifier stage 224, and theoutput of this amplifier stage 224 is provided to another of the A/Dconverters of the microprocessor sub-assembly 210 at a point 216 on FIG.5. This represents the current being drawn by the power oscillator(i.e., the inverter 250 depicted in FIG. 6).

The microprocessor circuit of the microprocessor sub-assembly 210outputs a digital signal at 218 that comprises pulse-width modulateddata, also referred to as “PWM data.” This pulsed signal travels througha transistor inverter/level-shifting/biasing circuit to drive the gateof the FET transistor 232. This PWM signal 218 consequently controls theduty cycle of the load current that travels from point 230 through the“sense” resistors 234 and 236. An inverter overload signal is providedat 222 when the above-mentioned SCR is latched, and an LED 240 can beprovided to give a visual indication of an overload condition to thehuman user.

Referring now to FIG. 6, an electrical schematic diagram 250 is providedshowing some of the components for an inverter sub-assembly for theinduction heating tool 10. A 120 VAC input power supply is used andprovided through a fuse and switch circuit at 252. Switch 252 isreplaced by a jumper wire when any control circuit is employed. Athermistor 254 is provided in this power circuit, as an inrush currentlimiter. The 120 VAC is provided to a rectifier and capacitor stage at256 to convert the voltage from AC into direct current.

A multiple winding transformer 260 is provided as a feedbacktransformer. One of the windings of this transformer 260 carries some ofthe current provided to the work coil 264, although most of the reactivecurrent that runs through work coil 264 is shared with the capacitors266 on FIG. 6, which generally correspond to the capacitor boardsub-assembly 44 on FIG. 2. In this manner, the coil of the transformer260 does not need to carry the full AC current that travels through thework coil 264.

The waveform on the output leg of an inductor 258 is similar to afull-wave-rectified, unfiltered waveform. The excursions at thisinductor lead, which feeds the center tap of the feedback transformer260, are from nearly zero volts, a level that is reached at the timesthe FET's 268 are switching, to about 209 volts, the level reached whenthe work coil peak-to-peak voltage is maximum.

The FET drain leads have waveforms similar in shape to the output leg ofinductor 258 except that every second “hump” is replaced by a value ofnearly zero volts as the corresponding FET 268 is driven into an “ON”condition for almost a full half cycle.

At the time the output of inductor 258 is at its peak of 209 volts (inan unloaded tool) and, as mentioned above, one of the FET's 268 is ON(i.e., conducting current), one side of the work coil is forced to be atzero volts with respect to the circuit ground. The other side of thework coil is then about 209 volts above the center tap of the feedbacktransformer 260 primary, or at about 418 volts above ground. Thisproduces a maximum voltage of about 418 V peak across the work coil, avoltage that diminishes to zero at the time the FET's 268 are made toswitch OFF. But since normal operation of the oscillator causes the endsof the work coil to be pulled to ground sequentially, this produces avoltage doubling effect at the work coil.

Referring now to FIG. 6, note that the schematic diagrammaticallydepicts a work coil 264 that is connected in parallel to the feedbacktransformer's effectively-center-tapped primary, the FET drain leads,and the bank of resonating capacitors 266 that provides the high currentlevels for the work coil. The work coil has two ends, and for the firsthalf of a given sine wave cycle, the first end is grounded through theFET 268 attached to it while current, from inductor 258, feeds theprimary center tap and flows through half of this primary to the firstend. While this occurs, the other side of the primary (of transformer260) is caused, through autotransformer action of the primary, to riseto a value that is always about twice the voltage of the center tap. Inthis way, the second end of the work coil 264 is brought to a peakvoltage of about 418 volts at the midpoint of this half cycle.

As discussed elsewhere herein, the coil/capacitor combination comprisesa high-Q tank circuit making the driven impedance, seen by the secondend of the feedback transformer 260 primary, quite high. During thishalf of the cycle, the second end has increased in voltage with respectto the first end from zero to about 418 volts, and then decreased toabout zero volts. At that point, however, the second end is thengrounded through the other FET 268, and current from inductor 258 (whichhas reached a maximum at this time) begins to flow through the otherhalf of the primary to the second end; through similar transformeraction, the voltage at the first end (with respect to the second end)rises from zero to 418 volts and then back to zero to complete thesecond half of this cycle.

During the second half of this cycle, the first end goes positive withrespect to the second end (or the second end becomes negative withrespect to the first end). Thus, if the coil voltage is measured with afloating oscilloscope having its ground lead attached to the first end,the waveform is very nearly a pure sine wave, in which the voltagewaveform is about 836 volts peak-to-peak, across the tank circuit ofcapacitors 266 and work coil 264.

The primary center tap exceeds the DC input level during its excursions.This is caused by the fly-back action of inductor 258 as the currentthrough it is continued for a short time after the voltage across it hasreached zero, about 45° into this cycle. This forces the output ofinductor 258 to become more positive than its input, thus elevating thecenter tap voltage even farther until the magnetic field around inductor258 has completely collapsed. The value of inductor 258 is generallychosen to have the correct inductance to supply the needed current tothe primary center tap at the 90° point in the cycle, to yield the bestvoltage peak for optimum circuit operation.

A fan motor 270 is driven from the low voltage side of the transformer260, through a diode circuit 272. This fan can be optionally provided tocool the work coil 264 inside the head 50 of the induction heating tool10 of the present invention.

Referring now to FIG. 7, an electrical schematic diagram 280 is providedshowing the output stage of a 50% duty cycle inverter circuit used tocreate 160 volts DC from a battery input. The battery current runsthrough a fuse to the center tap of a transformer 286. The transformer286 is used to generate the much greater voltage level required tocreate the +160 VDC output at 282. On the high voltage side oftransformer 286 is a set of rectifier diodes 284, as well as an inductorand capacitor filter circuit at the +160 VDC output at 282.

On the low voltage side of transformer 286 is a pulse-width modulatorcircuit controlled by a PWM controller chip 290. The power switchingtransistors are FET devices, depicted at 288, along with their biasingresistors and filter capacitor. An LED indicator 292 is provided whichilluminates when the battery voltage is sufficiently high. In apreferred embodiment of this circuit, the battery output voltage is 16VDC, and the LED indicator 292 remains ON so long as the battery'svoltage remains above 14 VDC.

A voltage comparison and hysteresis circuit at 294 enables and disablesthe PWM controller chip 290, switches the current for LED 292, and alsosets the hysteresis for this voltage level indication. Using the valuesindicated on FIG. 7, the hysteresis is set to about 1.5 VDC. This meansthat in a circumstance where a battery undervoltage condition isdetected (i.e., the battery voltage falls below 14 VDC), the inverteroutput is disabled, and remains “locked out” until the battery voltagelater rises to above 15.5 VDC.

The battery-supplied inverter circuit provides the +160 VDC needed forthe high voltage requirements of the present embodiments of theinduction heating tool 10 that are described herein. It will beunderstood that other power converter circuits could be used to create a+160 VDC output from a +16 VDC input voltage, and further that allsupply voltages described herein could be significantly altered in valuewithout departing from the principles of the present invention.

FIG. 8A is a block diagram 300 of the major electrical components of thefirst embodiment induction heating tool 10 of the present invention.Starting with a power source of 117 VAC (also sometimes referred toherein as a 120 VAC alternating current source) at 302, the line poweris connected to a fuse at 304, then an ON-OFF switch at 306, and athermistor at 308. This line voltage then drives into a transformer 316and also into a 160 VDC power supply 314.

The heating induction tool 10 of the present invention can also bebattery operated, and in that circumstance there would be no 117 VAC (or120 VAC) source. Instead, a battery 310 is utilized, which providesdirect current into an inverter stage at 312 (see FIG. 7, for example).This becomes the power source for the 160 VDC power supply 314. Eitherthe battery 310 or the transformer 316 provides power for a +5 VDC powersupply 318. This +5 volt supply provides power to a microprocessor stage350.

The output of the 160 VDC power supply drives a power oscillator stage322, which is the DC-to-AC inverter. A single printed circuit board 320can contain both inverter 322 and a switching transistor 324, which isequivalent to the FET circuit at 232 on FIG. 5.

The output of inverter 322 drives a work coil and a set of powercapacitors, which in combination are a tank circuit designated by thereference numeral 340. A temperature sensor at 342 (referred to hereinas an RTD, or Resistive Temperature Detector) is provided at the workcoil, and the output of the temperature sensor 342 is directed to abuffer circuit 344, which in turn drives an input of the microprocessorcircuit 350. This typically would be an analog signal, so an A/Dconverter is required, either in the buffer circuit 344 or on board themicroprocessor stage 350.

The first embodiment induction heating tool utilizes a multiple-segmentbar graph display 356, and also uses multiple EEPROM memory chips 358.In addition, this first embodiment tool uses an RS-232 serialcommunications port at 352, which has an RJ-11 jack at 354. This allowsthe induction heating tool to be in communication with a remotecomputer, such as a PC at 360 on FIG. 8A.

Referring now to FIG. 8B, a block diagram 301 is provided illustratingsome of the major electrical components of the second embodiment handheld induction heating tool 10. An AC power source, such as 120 VAC linevoltage, is provided to supply power to a rectifier and filter stage at370. A DC—DC pre-regulator 372 receives direct current from the outputof the rectifier/filter stage 370, and a DC-AC inverter 374 receives acontrolled voltage level from the pre-regulator 372. The output of theinverter 374 is used to drive an induction head 376, which isessentially the same as the head sub-assembly 50 depicted on FIGS. 1 and2.

The induction head 376 generates a magnetic field via a work coil (notshown on FIG. 8B, but which is part of the induction head), and thismagnetic field is directed toward an aluminum susceptor 390 whichincludes at least one relatively thin “foil” layer of electricallyconductive aluminum (but which could easily use a different material forits electrically conductive “foil” layer or layers).

A controller 380 utilizing a microprocessor is provided to detect thevoltage and current parameters 382 at the input of the pre-regulator372. Controller 380 is responsive to the sensed parameters 382, andgenerates a pulse-width modulated (PWM) control signal 384, havingproperties determined by the controller. Other methodologies could beused other than a PWM control signal, and moreover, the controller coulduse a logic-state machine in lieu of a microprocessor, if desired.Furthermore, the entire interface and control circuit could beconstructed entirely of analog components, which is an alternativeembodiment described in reference to FIG. 16.

Referring back to FIG. 8B, during operation the rectifier/filter stage370 receives AC power from the AC power source 302. The DC—DCpre-regulator 372 pulse-width modulates the DC power signal from thestage 370 to provide a DC “power” signal of the proper magnitude toallow the DC-AC inverter 374 to energize the induction head 376 withsufficient high frequency AC power to induce heating in the thin foilAluminum susceptor 390.

In one mode of the second preferred embodiment, the DC-AC inverter 374operates at a nearly fixed frequency (typically in the kilohertz range).The sensed voltage and current at 382, which is at the input of thepre-regulator 372, is fed “forward” to the controller 380, which usesthe sensed voltage and current to determine the proper operatingparameters of the induction head 376 and provides the DC—DCpre-regulator 372 with control instructions or commands 384. The voltageand current produced by the pre-regulator 372 is varied, as necessary,to keep the input power (i.e., the input voltage and current beingdetected at 382) at a substantially constant value.

As described in the flow chart of FIGS. 17A-17D, the substantiallyconstant input power consumed at the pre-regulator 372 is sufficient togenerate a magnetic field at the work coil (of the induction head 376)so as to quickly induce substantial eddy currents in the foil susceptor390, thereby causing the susceptor to quickly rise in temperature. Theamount of time that the eddy currents are induced by the magnetic fieldis controlled by controller 380, and this time is limited to an intervalthat both insures that a “good” bond is created by the adhesive affixedto the foil susceptor 390, and insures that the foil is not overheatedto a point that it entirely melts (or explodes), which potentially couldcause the adhesive material to burn.

An example circuit constructed according to the general principles ofthis block diagram is illustrated on FIGS. 14A-14B, and this circuit isdescribed in greater detail. It will be understood that portions of thisblock diagram of FIG. 8B could be significantly modified withoutdeparting from the principles of the present invention. For example, thepower supply could be a battery, or even a solar panel. On the otherhand, the “feed-forward” characteristics of the block diagram, by use ofthe current and voltage information provided at 382, is a unique designin the field on induction heating tools, which allows the presentinvention to forego the use of more expensive sensors, such as a currenttransformer to detect the high currents of the induction head.

While the above discussion of FIG. 8B describes a “feed-forward”configuration for controlling the power supplied to the induction head376, it will be understood that a “feedback” configuration couldnevertheless be used in accordance with some of the other principles ofthe present invention. The “ramp control” and “distance detection”aspects of the present invention (which are described below in greaterdetail) are novel features that could be combined with a feedbackcontrol system to effectively operate the induction head 376 (whichcontains the work coil). For example, on FIG. 8B the voltage and currentsensing information comes from the input side of the pre-regulator 372,while the control action takes place “downstream” from that point (i.e.,at the control signal 384), thus making this a “feed-forward” device.

However, if the voltage and current sensing information were to insteadcome from a location that is downstream from the control point (i.e.,the control action would then take place “upstream” from the sensingpoint), then a “feedback” configuration would result. This could easilybe done by looking at the voltage and current at other locations in thecircuit, such as the output side of the pre-regulator 372, the outputside of the inverter 374, or the output side of the induction head 376.One word of caution is in order; the power input to the pre-regulator372 can quickly increase if one is using such a feedback configuration.This increasing power can become quickly destructive if steps are nottaken to control the current input at the pre-regulator 372; by the timethe feedback information has been provided to the controller 380, itcould be too late to prevent an overcurrent condition at thepre-regulator input. Therefore, it is best to provide some type ofcurrent limiting circuitry when using the feedback configuration.

In eddy current induction heating applications where the heated piece,or susceptor, cannot be placed inside the work coil where the magneticfield strength is maximum, power transfer efficiency is reduced. In thepresent invention, the susceptor foil is quite thin, and relativelylittle power is required to create significant eddy currents that resultin development of high temperatures.

The heating of a thin susceptor may be understood in terms of thecurrents induced in it by the time-varying magnetic flux from the drivehead pole piece. From Maxwell's equations for electromagnetic fields,the electric field induced by a time-varying magnetic flux is given by:

∫E•d1=dΦdt,

where the integral of the electric field E is taken around any closedpath in space and Φ is the magnetic flux linked through that path. InFIG. 12, the flux 424 generated by a simple cylindrical electromagneticpole 420 is illustrated. It is cylindrically symmetric around the axisof the pole. The induced electric field lines 400 are thereforecylindrically symmetric, as shown in FIG. 9 which is viewed looking downon the centerline of the pole piece.

When a relatively thin susceptor 430 is placed in the region near thepole piece as shown in FIG. 12, the magnetic flux penetrates through thesusceptor, and an electric field is induced in the susceptor accordingto Maxwell's equation. The electric field causes circulating, or eddy,currents to flow in the susceptor. The current density is a product ofthe electric field and the resistivity of the conductor material. Thecurrent density can be represented as a vector in the same direction asthe electric field. If the susceptor is wide with respect to themagnetic flux distribution and perpendicular to the axis of the polepiece, then the electric field 400 and the resulting current densitydistributions will be nearly circular, as shown in FIG. 9.

In FIG. 12, a cylindrical ferrite rod 420 is magnetically driven by analternating current flowing through a wire-wound coil 422 wrapped aroundthe rod. The combination of the ferrite rod 420 and coil 422 comprisesone implementation of a “work coil.” The susceptor 430 is considered tobe an infinite uniform sheet at the distances of this discussion. Thesusceptor 430 is illustrated in this view edge-on, and is spaced apartfrom the proximal end of the rod 420 at a distance of about one-eighthinch (about 3 mm). When an alternating current is driven through theelectrically conductive coil 422, a magnetic field is created,represented by “lines” of force generally indicated by the curves 424.

In FIG. 9, the flux linked by closed circular paths near the centerlineof the pole piece will link only a small magnetic flux, and theresulting electric fields and current densities will be low—approachingzero at the centerline. For closed paths of larger radius, the fluxlinked will be larger and the current density will also be larger. Amaximum is reached at the closed path having a radius approximatelyequal to the pole piece radius at about the point where the flux beginsto return in the reverse direction through the susceptor (near thecrossed dot symbol {circle around (x)} in FIG. 12) and, from that pointout the electric field and current density will decrease. A notionaldistribution of the electric field and current densities 426 is shown inFIG. 13. The power dissipation in the susceptor varies as the currentdensity squared and is proportional to the resistivity of the susceptormaterial. The susceptor is thus heated in a ring or “donut” pattern thatreflects the electric field distribution induced by the fluxdistribution.

If the susceptor is relatively narrow with respect to the magnetic fluxdistribution (see FIG. 10), then the induced electric current must allreturn along the edge of the susceptor 404, and the electric field lines402 become somewhat distorted, as shown in FIG. 10. The current densityalong the edge can be significantly higher than it is elsewhere in thesusceptor 404, and the edge is preferentially heated.

To diminish uneven heating, the oval current paths 402 can be modifiedto have resistivities in the susceptor regions between the edges and thecenter that are made higher by the introduction of small holes oropenings 414 (see FIG. 11) placed along the length of a susceptor 412.The above-noted Boeing patents use openings in susceptors to addressthis issue. The electric field lines 410 are not greatly affected nearthe center of the susceptor, e.g., the lines at 408 or closer to thecenter.

Closer to the edge, however, the current densities are somewhat reducedby the greater resistively created by holes 414, in the area generallydesignated by the reference numeral 416. To accomplish this, currentdensity reduction is desired from about the third representative ring408, outward. For one approach, this requires that holes 414 be placedalong the length of the strip in parallel lines, but not near the edge418 where increased resistivity would cause even further heatingdisparity across the width “W” of susceptor 412. Another approach is tolocate a single row of holes/openings along the centerline of susceptor412.

Referring now to FIGS. 14A-14B, an electrical schematic 500 of a secondpreferred embodiment of the present invention is illustrated. Linevoltage power from a 120 VAC electrical outlet enters a printed circuitboard (not shown in a structural drawing) at terminals E1 and E2, thenthrough a fuse F1, and a current limiting thermistor RT1, and across avaristor V1. This is the beginning of a power input circuit 510. The ACpower is filtered by capacitors C1-C6 and an inductor L1 to removedifferential-mode noise and common-mode noise. A full bridge rectifierBR1 rectifies the filtered 120 VAC, which is again filtered by a bulkcapacitor C27 to remove much of the 120 Hz ripple.

A regulated positive 5 VDC power supply is provided at 516, and mainlyconsists of a standard linear voltage regulator chip at Q5. Its inputpower supply is the +12 VDC rail that drives most of the analog chips inthe circuit 500. Polarity protection diodes D16 and D17 are provided, aswell as a filter capacitor C22 at the +5 VDC output rail. In anexemplary construction, additional filter capacitors C18-C21 areprovided across the +12 VDC rail at various physical locations on theprinted circuit board of this second preferred embodiment.

The voltage across C27 forms a DC input voltage to a buck converter 514which is formed by a switching MOSFET Q1, a free-wheeling diode D1, aninductor L2, and a filter capacitor C7. The MOSFET Q1 switches atapproximately 75 kHz and has a controlled duty cycle varying from 0% to100%. The voltage across C7 is equal to the MOSFET duty cycle times thevoltage across C27. A duty cycle of 0% implies that the switch iscompletely off and the output voltage across C7 is 0V. A duty cycle of100% implies that the switch is completely on and the output voltageacross C7 is ideally equal to the voltage across C27. At 100% dutycycle, the buck converter supplies about 160 volts DC at the terminalsE5 and E6 (across C7). At lower duty cycles, the apparent voltagesupplied by buck converter 514 will be less than 160 VDC, and thisapparent voltage will be substantially equivalent to 160 volts times theduty cycle at a particular time interval.

It will be understood that the voltage output from buck converter 514 isa variable DC voltage. However, it will be understood that anappropriate output could be provided using other circuit topographies ofpower converters, without departing from the principles of the presentinvention.

When the MOSFET Q1 turns on, current flows from C27 through the inductorL2, thereby charging C7 and also flowing through the load (at terminalsE5 and E6) parallel to C7. The return path to C27 is through currentsense resistors R21 and R30. When the MOSFET Q1 turns off, the storedenergy in the inductor L2 discharges and a free-wheeling current flowsthrough C7 and the parallel load in a circular path through thefree-wheeling diode D1 until the MOSFET Q1 turns on again. Thisrepetitive action provides a variable DC output voltage at the output ofC7 and across the load at E5-E6 (which are the same connecting points asJ3 and J6 on FIG. 15).

The MOSFET Q1 is controlled by a combination of analog and digitalcircuitry comprising a 75 kHz clock at 522, an integrator 530, acomparator 532, and a high-current MOSFET gate driver 534. The 75 kHzclock comprises a Schmidt Trigger inverter U1A, a timing capacitor C14,and a timing resistor R13. The resulting square-wave signal is modifiedby C15, R14, and D9 to provide 75 kHz spikes that trigger a transistorQ4 to reset the integrator 530 comprised of an op-amp U2A, an integratorcapacitor C16, an integrator resistor R17, and bias resistors R15 andR16.

The output of U2A is a 75 kHz saw-tooth waveform that is used as thepositive input to the pulse-width-modulating comparator 532 that uses anop-amp U2B. The negative input to pulse-width-modulating comparator 532is an analog signal between 0V and 5V that is generated by amicroprocessor U3 (at 540), and filtered by R18 and C17. The comparisonof the microprocessor-generated analog signal and the saw-tooth waveformprovides a 75 kHz square wave at the output of pulse-width-modulatingcomparator 532 that has a duty cycle proportional to the level (ormagnitude) of the analog signal. This square wave output signal isbuffered by Schmidt Trigger inverter stages U1B-U1F at 534 to provide ahigh-current square-wave drive signal to the gate of MOSFET Q1.

The DC voltage across C27 is scaled by resistors R26 and R27 andfiltered by a capacitor C26. A Zener diode D15 limits the voltage acrossC26 to 5.1 VDC in case of a surge condition. The voltage across C26 issensed by an input port on the microprocessor U3. The current flowingthrough the MOSFET Q1 is also flowing through resistors R21 and R30,while a resistor R22, and capacitors C23, and C24 filter the voltagedrop across the combination of R21 and R30 before being directed to anop-amp U2D.

An amplifier stage 520 comprising op-amp U2D, and resistors R23 and R25multiplies the filtered voltage from the current shunt by a gain offive. A resistor R24 and capacitor C25 filter the current-sense signalagain. A Zener diode D14 limits this voltage in case of a high currentcondition. The current-sense signal is then sent to an input port on themicroprocessor 540. Microprocessor 540 (U3) detects the current andvoltage at the input to the buck converter 514 to predict and/orcharacterize the power supply and to provide the proper analog controlsignal to the pulse-width-modulating comparator U2B, at 532.

An unused stage of the quad op-amp chip U2 is depicted at U2C. Itspositive input is connected to DC common, and its negative input isdirectly connected to its output.

Two temperature sensor inputs are provided to the microprocessor 540, inwhich thermistors (not shown in the schematic 500) are connected atterminals E7-E8 (for the first thermistor) and E27-E26 (for the secondthermistor). These temperature analog signals are signal-conditioned byR29 and C28 for the first thermistor, and R41 and C39 for the secondthermistor. One thermistor monitors the temperature of the head 50 ofinduction heating tool 10, while the other thermistor monitors theambient air temperature at the case of tool 10.

The processing circuit on FIGS. 14A-14B comprises the microprocessor 540(U3), along with a crystal oscillator X1, and capacitors C32 and C33,which make up the clock circuit 542. Also included in the processingcircuit are various other interfacing or filter resistors andcapacitors, such as R33, C31, C30, R32, and C29. Further, a filterconverts a digital output from the microprocessor 540 to an analogsignal used to control the duty cycle of the MOSFET Q1. This filter ismade up of resistors and capacitors R34, R35, C35, and C36.

A memory circuit 546 is also provided for use by the processing circuit.In this second preferred embodiment, a serial EEPROM designated U4 isused. This single chip contains sufficient memory storage capacity tohold the important variables utilized by the induction heating tool,although certainly additional memory chips could easily be added tostore greater amounts of operating data, if desired. A small filtercapacitor C34 is provided on the +5 VDC power rail at the EEPROM chip.

It will be understood that the microprocessor 540 could be provided withon-board memory, such as EPROM or EEPROM to store operating parametersand perhaps some accumulated data. In such a microprocessor, the serialEEPROM memory chip U4 could be eliminated in models of the heating toolthat do not require historical operating data to be stored and laterdownloaded to a host computer.

A serial communications port 560 is provided, comprising an infraredserial interface chip U5, and associated interface passive componentsR40, C37, and C38. The use of this type of infrared communications portallows a host computer (e.g., a PC or workstation) to be connected tothe induction heating tool 10 without any possibility of transferring anundesirable voltage into the host computer while the tool 10 isoperating on line voltage.

A variable resistor R31 (e.g., a potentiometer) is provided for setuppurposes. The circuit power operation can be manually controlled by thispot R31 without the microprocessor even being installed in the circuit,if desired for experimentation or setup purposes. Once production unitsare being assembled, pot R31 may well be eliminated from the circuitboard if it becomes redundant.

A power control potentiometer P1 is provided so the user can adjust theoutput power by about ±20%, and this adjustment can be made between each“shot” of the induction heating tools, if desired. An FET transistor Q9at 524 is provided as a safety shutdown device. Q9 can disable the PWMclock at U1A, if necessary.

The microprocessor 540 also controls four digital outputs at 544, 550,552, and 554. These outputs (via an FET transistor, part number IRLL014)drive: (1) the fan 56 (see FIG. 2); (2) the tactile feedback solenoid orbuzzer device 26 (see FIG. 2); (3) the LED's at 34 on FIG. 2, whichprovide status and “fault” information; and (4) the LED's at 64 on FIG.2, which provide illumination of a work piece for a user working in adark room. The status or fault LED's 34 are typically provided in red,while the illumination LED's 64 are typically provided in both blue andyellow colors.

The signal line at pin 3 of microprocessor 540 carries an analog signalthat represents the input voltage magnitude of the buck converter 514.This information is used in the software control program, as discussedin reference to FIGS. 17A-17D, at the step 758. The signal line at pin 4of microprocessor 540 carries an analog signal that represents the inputcurrent magnitude of the buck converter 514. This information is used inthe software control program, as discussed in reference to FIGS.17A-17D, again at the step 758.

It will be understood that many of the circuit components found in theschematic diagram 500 could be replaced by a logic state machinecircuit, or other similar logic device available today, or available inthe future. Such a substitution of components or an enhancement of logicand interface components is contemplated by the inventors. Moreover, theentire circuit could be implemented with an analog circuit, an exampleof which is discussed in detail below, with reference to FIG. 16.

Referring now to FIG. 15, the current-source parallel-resonant inverterpower oscillator stage comprises an input current smoothing choke L1,MOSFET transistors Q1 and Q2, and a feedback transformer T1 whichprovides a voltage-multiplying function to increase the resonant tankvoltage and also provide dual inverted MOSFET gate drive signals. TheMOSFET's Q1 and Q2 each require a series resistor-capacitor snubbingcircuit. For Q1, the snubbing circuit 572 includes C5 and R9; for Q2,the snubbing circuit 574 comprises C4 and R10. It will be understoodthat, if desired, these “power” components could be mounted on aseparate printed circuit board (or other structure) from the “logicboard” described in the schematic drawing of FIGS. 14A-14B that containsmainly low power components.

The gate drive signals from the transformer T1 of the inverter poweroutput portion 570 are conditioned by the bias networks comprisingD1-D3, C1-C3, R1, and R3-R8. R8 is an adjustable resistor element, suchas a potentiometer at 580, and D3 is a Zener diode used at 582 to createa bias voltage source.

A set of bias resistors R11 and R12 provide extra bias voltage to theMOSFET gates when the DC input voltage to the oscillator is low to aidin the starting of the oscillator. The DC input to the inverter entersthrough terminals (on this board) J3 and J5. The resonant tank isconnected between terminals J1 and J2.

The resonant tank circuit is made up of a high qqality-factor inductor590 in parallel with a capacitor (or capacitors) 592. The inductor 590is the “work coil,” which transfers energy to the susceptor (not shownon this drawing). The values of the inductor and capacitor(s) arechosen, in one configuration of this second preferred embodiment, toachieve a 130 kHz resonant frequency in order to more effectivelytransfer energy to the chosen susceptor. In this embodiment, the overallcapacitance is about 0.35 μF, and multiple physical capacitors are usedin parallel with one another (see the capacitor board 44 on FIG. 2). Thework coil 590 on this schematic diagram of FIG. 15 essentially comprisesthe electrically conductive windings 52 (on FIG. 2) of Litz wire.

It will be understood that different component values and types ofelectronic logic gates and analog stages could be used in the circuitsof FIGS. 14 and 15 without departing from the principles of the presentinvention. As stated above, the illustrated embodiment comprises asecond preferred embodiment, and many other, but similar, embodimentscould be constructed that would operate in a similar manner.

It will be further understood that the operating frequency stated aboveof 130 kHz is a desired operating frequency for a particularconstruction of the second preferred embodiment of the presentinvention, however, this is a “design frequency” only and whenproduction units are built, their actual operating frequencies will notlikely be exactly 130 kHz, due to component value variations if for noother reason. Moreover, the induction heating tool of the presentinvention is able to operate over a very wide range of frequencies (suchas below 1 kHz to greater than 1 MHz), without departing from theprinciples of the present invention. It is contemplated that variousstyles of susceptors could be effectively actuated by a single inductionheating tool producing a magnetic field at a single output frequency,however, it also is contemplated that certain styles of susceptors maywork better with one or more induction heating tools that operate(s) atmore than one frequency to induce the eddy currents in the susceptors.In this circumstance, the operating head 50 (see FIG. 1) could be madeto be interchangeable, if desired, to change the output frequency of theinduction heating tool.

Referring now to FIG. 16, an analog controller circuit design isprovided in a block diagram format. Pulling a trigger 610 (which isequivalent to the trigger 22 on FIG. 1) initiates the firing sequence. Atimer 614 is activated and an input control device 612 for an integrator620 applies a voltage to the input of this integrator 620. These actionsstart a voltage ramp at the output of integrator 620 which passesthrough a pulse-width modulator 622 and then to the voltage control of apre-regulator circuit (not shown on FIG. 16) by way of a PWM signal at624. The pre-regulator circuit (not shown on FIG. 16) provides an inputvoltage to be used by the output (power oscillator) inverter (not shownon FIG. 16).

During the ramping process, a comparator 644 monitors a “current sense”signal 650 from the pre-regulator input. An adjustable resistor (e.g., apotentiometer) 646 is provided to set a “ramp stop” threshold voltage.When the voltage level of the current sense signal 650 reaches theramp-stopping threshold voltage, the output of the comparator 644 placesthe integrator 620 in a “hold” condition, at the present output voltage.This action signals the timer 614 to initiate the “power-tracking” modeof operation by enabling another integrator 634 in a sampled mode.

Integrator 634 is used to integrate the power sample generated by amulti-quadrant multiplier 630. The output of the multiplier 630 alsofeeds another comparator 640, which directs the input control 612 toincrease or decrease the input voltage to integrator 620. This actionregulates the power supplied by the pre-regulator (not shown) to theoutput (power oscillator) inverter (not shown), which in turn controlsthe power delivered to the susceptor (not shown on FIG. 16). Anadjustable resistor (e.g., potentiometer) 642 is provided to set a“power track level” threshold voltage.

The present output-state of the first integrator 620 is also directed toa non-linear function generator 632, which generates a non-linearthreshold voltage for a third comparator 636 that is used to terminatethe firing cycle when sufficient energy has been delivered to thesusceptor. The output of the comparator 636 disables the input control612.

Depending upon the output level of the first integrator 620, thepulse-width modulator 622 will operate at 100% duty cycle or at a lowerduty cycle percentage, according to a duty cycle controller 626.Duty-cycle controller 626 energizes the pulse-width modulator 622 at agiven duty cycle depending upon the output level of the first integrator620. This lengthens the total time of power application for susceptorsthat are in close proximity to the work coil. At least one adjustableresistor (e.g., a potentiometer) 628 is provided, which can be used toset a minimum duty cycle threshold voltage, and/or which can be used toset a maximum duty cycle threshold voltage.

Operating in the background during the tool heating cycle, is a watchdogtimer output at 616, and also an over-current detector 654. The purposeof the watchdog timer is to control the maximum run time of a cycle inthe event that the predetermined total energy limit is not reached. Theover-current detector 654 disables the input control 612 in the eventthat the current-sense signal 650 from the pre-regulator exceeds apredetermined maximum limit. This predetermined maximum current limit isset by an adjustable resistor (e.g., potentiometer) 652.

FIGS. 17A-17D are a flow chart illustrating the major logical operationsthat are performed by the microprocessor circuit 540 on FIG. 14 of thesecond preferred embodiment of the induction heating tool of the presentinvention. It will be understood that a logic state machine could beemployed to perform most, or all, of these logical operations in lieu ofa sequential processing circuit, such as microprocessor 540. Of course,the logic state machine could be included in an integrated circuit thatcontains many of the other electrical components needed for interfacingto external digital or analog input and output signals.

At a step 700, power is applied by the connection of the tool to astandard 120 VAC outlet, or to a battery pack. Application of the powercauses a supply voltage to be applied to the control microprocessor 540,which undergoes a reset sequence (using a power-on reset circuit that isbuilt into the microprocessor 540), and then begins the code executionwhen the applied power is within limits and the oscillator has started.

An initialization procedure now occurs at a step 702. During thisoperation all of the input/output ports are defined and initialized,variables are assigned, and various configuration parameters aretransferred from EEPROM. This process is not repeated unless the poweris removed and reapplied.

After initialization, the software program controlling themicroprocessor enters a main “waiting loop” where four differentconditions are monitored. The first condition is a check of the headoperating temperature, which occurs at a decision step 704. The coolingfan is turned on or off, depending upon the current temperature of thehead. The on and off limits are user definable and stored in the EEPROM.If the result at decision step 704 is YES, in which case the headtemperature is “high,” then a step 708 turns the fan ON. Otherwise astep 706 turns (or leaves) the fan OFF. In both cases, the logic flow isdirected to a decision step 710.

At decision step 710, the “work light timer” is examined to see if thelight should be on or off. Both the enable/disable function, as well asthe run time, are user-definable and stored in the EEPROM. If the resultat decision step 710 is YES, in which case the work light timer hasexpired, then a step 714 turns the work light OFF. Otherwise a step 712turns (or leaves) the work light ON. In both cases, the logic flow isnow directed to a decision step 720.

At decision step 720, the program checks to see if there are anyrequests from the communications port 560. Requests from a remotecomputer may be for reading configuration or firing data, or in writingnew configuration data to the EEPROM 546. If a request to service thecommunications port exists, then a step 722 will do so; otherwise thelogic flow immediately is directed to a decision step 724.

At decision step 724, the program monitors the trigger 22 to see if ithas been pulled by the user. If not, the program loops back to step 704to check the head temperature, etc. If the trigger has been pulled theprogram proceeds to a “running” mode, starting at a decision step 726.

Before the induction heating tool 10 begins its “firing” sequence, thetemperature of the head is measured and decision step 726 determines ifan over-temperature condition exists. If the head temperature is toohigh, even though the fan may be running, the tool 10 will not fireuntil the temperature has dropped within limits. In that circumstance, afault indication occurs at a step 728, and one of the LED's on the toolcan be illuminated. On the other hand, if the head temperature is withinrange, the logic flow is directed to a step 730.

At step 730, the program reads the power-scaling pot and calculates ascale factor and various limits. The buck converter 514 (which could bea different type of power converter) is then enabled at a step 732, andthe PWM (pulse-width-modulated) signal is set to output its startingvoltage (e.g., 30 volts, or 50 volts). A ramping function is commenced,and the “run” lamps (LED's) are illuminated.

During the ramping operation the program monitors the DC current drawnby the buck converter and compares this at a decision step 734 to a ramp“stopping current.” This is a limit that is user definable andpreferably is stored in the EEPROM. If decision step 734 determines thatthe stopping current has been reached, then a step 736 starts a logical“interval timer,” as discussed below. In one mode of the secondpreferred embodiment, the ramp stopping current is set to 1.7 Amperes,and the “ramp time” (i.e., the desired time interval during which thecurrent-ramping from zero (0) to 1.7 Amps occurs) is about 50 msec.

On the other hand, if the stopping current has not been reached, theprogram checks at a decision step 740 to see if the DC current is aboveor below the normal limits. If the DC current is out of range, theprogram declares a fault at a step 744, turns off the buck converter,and gives a visual indication (at one or more of the LED's). The faultstatus is also stored in the EEPROM on a firing-by-firing basis.However, if the DC current is within its normal limits then the programincrements the PWM at a step 742, which increases the voltage to theinverter. This process continues (in a loop through steps 734, 740, and742) until the DC current is equal to or greater than the ramp stoppinglimit, at which time the program transitions to a “power track” mode.

To begin the power track mode, the interval timer is started at step 736which will limit the overall run time of the firing and provide intervaltiming for the energy calculations. This interval timer operates with a10 msec interval; the interval timer can run many times throughout asingle heating operation, and could run using an 8-bit counter, forexample, 255 times before the counter reaches a hexadecimal value of FF,thereby providing a “total time” limit of 2.55 seconds. A decision step750 examines the relative position of the head to the susceptor todetermine if the susceptor is sufficiently “close” to the head. If thesusceptor is within a specified distance from the head, a step 752 setsa flag which will be used to control the duty cycle of the appliedpower.

The program continues by monitoring the interval timer at a step 754.During the monitoring procedure, several samples of DC current and DCvoltage readings for the buck converter are summed (or otherwiseaccumulated) and stored at a step 758. Also during this interval, the DCcurrent is examined by a decision step 756 to determine if anover-current condition exists. If an over-current condition is detected,the program disables the buck converter at a step 760, and displays andstores the fault. However, if the current is within normal limits, theDC current and DC voltage summing operations continue until the timer'stime interval has expired (which is described below in greater detail).After the interval timer has expired the average values of DC currentand DC voltage are computed at a step 762. These values are then used tocalculate at a step 764 the present power draw and to accumulate thetotal energy for this operating cycle.

A decision step 766 next checks the susceptor “close” flag and a dutycycle flag to see if the power should be throttled back to its minimumvalue. If so, the logic flow is directed to a step 768 where power isreduced to the minimum value. If not, the logic flow is directed to adecision step 770. This full-power or minimum-power duty cycling can beused when the susceptor is quite close to the work piece, in order tosomewhat lengthen the heating cycle. Under normal circumstances, a veryfast bonding cycle is desirable; however, it is important to not allowthe work coil's magnetic field to overpower the susceptor to an extentthat the susceptor may literally melt very quickly and thus either burnthe adhesive or cause the adhesive to crystallize, thereby forming apoor bond with the work piece. A reduction of the power, andcorresponding lengthening of the heating cycle, is one method ofpreventing these occurrences.

During the full-power portion of the heating cycle, the “present power”is compared to the “tracking power” at decision step 770. The trackingpower attribute can be made user programmable, and stored in the EEPROM.Corrections to this attribute are made as required at a step 772, inorder to keep the power within the tracking limits. The voltage on thePWM (i.e., the duty cycle value, which ranges from count values 0-FFhexadecimal) is adjusted to keep the input power at a substantiallyconstant value while continuing to heat the susceptor. The adjusted PWMvalue is used to infer the susceptor distance during the next 10-mseccycle.

A decision step 774 compares the accumulated total joules to a “totaljoule limit,” which is stored in a look-up table in memory (which couldbe either the EEPROM or on-board EPROM within the microprocessoritself). This look-up table is addressed using the susceptor distanceinformation, which links the accumulated total joules and “total joulelimit” to the “susceptor distance;” the amount of joules for each10-msec cycle is based upon the look-up table value. The look-up tablevalues are typically derived empirically, and the “susceptor distance”terminology used in this description of operation is substantially equalto actual physical distance between a susceptor and the work coil whilein operation, at least for appropriate styles of susceptors.

Each 10-msec cycle will produce a “new” value for the susceptordistance/total joule limit attribute, although the result may be theexact same value for many consecutive cycles. The process of “adjusting”the output energy level (by “adjusting” either or both of the currentand/or voltage at a point in the power converter) could be simply amatter of maintaining the present output energy level for at least twoconsecutive cycles (or for “many” consecutive cycles, as noted above).On the other hand, the process of “adjusting” the output energy levelcould require a different output energy level for each consecutive10-msec cycle, even perhaps over the entire heating event of aparticular susceptor.

As noted above, the current and/or voltage adjustments can be made atthe input of the power converter (e.g., a buck converter, an oscillatorcircuit, or an inverter), while the electrical characteristics beingsensed (typically both the current and voltage) could be made at theinput of the power converter (in a feed-forward mode), or these sensedelectrical characteristics could be made at a downstream point of thepower converter (in a feedback mode), including at the power converter'soutput, or directly at the work coil.

It will be understood that a mathematical calculation could be made inlieu of using a look-up table to determine the susceptor distance/totaljoule limit value. If a calculation is used as the control methodology,it should be noted that the expression is a non-linear function,including an exponential component. The analog circuit embodiment (seeFIG. 16) also uses a similar non-linear function for this determination.

A decision step 782 determines whether or not the “total time” hasexpired, which is the time after the first 10-msec cycle commenced. Ifthe total joule limit has not been reached, and if the total time hasnot been exceeded, then the computer program continues tracking thepower, and the heating cycle continues by directing the logic flow backto step 754.

However, if the joule limit has been reached, a step 776 turns off thebuck converter, and a decision step 778 waits for a “hold timer” toexpire; the hold timer begins timing when power to the work coil isinterrupted, in situations where it is desired each cycle time to beidentical. After the hold timer has expired the run-light is turned offat a step 780, and the firing information is stored in the EEPROM.

The effect of steps 770, 772, 774, and 776 allows the induction heatingtool 10 to run at its maximum predetermined power, as measured by itsinput power at the buck converter, while adjusting its effective outputpower by varying the PWM duty cycle as needed to maintain its inputpower at the predetermined maximum level, until reaching a time durationat which it is determined that the accumulated total joules insure thata bond has been made by the susceptor.

A decision step 786 determines whether or not a “work light” has beenenabled by the user. If so, then a step 788 now turns on the “worklight” and the logic flow is directed to a decision step 790 whichdetermines whether or not the solenoid (or buzzer) has been enabled. Ifso, a step 792 now actuates the solenoid (or the buzzer) for theuser-programmable time duration that has previously been stored in theEEPROM.

The user is able to select a programmable time delay, used to control aminimum time interval between firings. A decision step 794 determineswhether or not this time interval (between firings) has yet occurred.The time interval “between” firings is based upon the end of the mostrecent previous firing and the beginning moment (or start) of the nextfiring. The software logic essentially waits for this time interval tooccur before continuing to the next logical operation.

The user is also able to utilize an “ironing” mode of operation whenusing the induction heating tool of the present invention. This isuseful when bonding two large sheets of material together, such asaffixing a Formica top to a wood surface for a piece of furniture. Adecision step 796 determines if the “ironing” mode of operation has beenselected by the user, and if so, the program immediately returns to themain loop (regardless of whether the trigger 22 is currently beingactuated).

On the other hand, if the ironing mode has not been selected by theuser, then the logic flow is directed to a decision step 798 whichdetermines if the trigger 22 has been released. If not, step 798 waitsfor the trigger to be released, and then the program returns to the mainloop at decision step 704.

FIG. 18 illustrates a double-foil susceptor 800, in which an insulativelayer 806 is provided between two different layers of foil at 802 and804. If the thin foil susceptor principle is utilized in theconstruction of this double-foil susceptor 800, then the magnetic fieldgenerated by the induction heating tool 10 of the present invention willproduce eddy currents in both of the foil layers 802 and 804. This canbe accomplished without requiring an increase in the magnetic field,which at first glance may be thought of as being required to induce themagnetic field in both foil layers. The main reason a larger magneticfield is not required is that the foil layers 802 and 804 are both quitethin, and the magnetic field fairly easily penetrates through thenearest foil layer into the farthest foil layer, thereby inducing eddycurrents in both foil layers simultaneously.

In essence, given a constant intensity (alternating) magnetic field, thedouble-foil susceptor 800 will absorb approximately twice the energythat a single foil susceptor of similar dimensions would otherwiseabsorb, thereby inducing a much faster temperature rise in the overallsusceptor by virtue of the additional eddy currents produced in thefarther layer. Since one of the advantages of the present invention isto cause the susceptors to heat up quite rapidly so that their outeradhesive surfaces will rapidly melt, or at least soften, then it can beseen that the double-foil susceptor 800 can aid in accomplishing thattask.

FIG. 19 shows a similar triple-foil susceptor design at 810, whichincludes two separate insulative layers at 820 and 822. Surrounding bothof these insulative layers on their large surfaces are parallel thinfoil layers 812, 814, and 816. Based upon the principle discussed abovewith respect to the double-foil susceptor 800, if the foil layers are ofa sufficiently small thickness, then the magnetic field produced by theinduction heating tool 10 of the present invention will penetrate intoall three of the foil layers 812, 814, and 816, thereby inducing eddycurrents in all three of these foil layers. When this actually occurs,the temperature rise is increased at a greater rate, therebyaccomplishing one of the main tasks of the present invention in causingthe outer layer of adhesive to rapidly either melt or soften so that itcan quickly join two structural members together in a permanent bond. Itwill be understood that there is no theoretical limit to the number ofseparately insulated foil sheets that may be used together in amultiple-thickness susceptor structure.

As discussed above in connection to single layer foil susceptors, boththe double-foil susceptor 800 and the triple-foil susceptor 810 can alsobe re-heated so as to allow a person to remove the same structure thatwas previously firmly bonded together.

FIG. 20 illustrates a (single-foil or multiple-foil) susceptor 830 thatincludes fusible portions or “links” that are depicted at the referencenumerals 834. As can be seen in FIG. 20, there are multiple holes(openings) or cut-outs at 832 in the susceptor 830. The closest distancebetween two of these holes or cut-outs 832 is the distance indicated bythe reference numeral 836. This distance could be equal throughout theentire susceptor, or it could be different from one set of cut-outs toanother set, if desired.

When the susceptor 830 is subjected to a magnetic field, its foil layerwill exhibit eddy currents that will cause the foil layer to increase intemperature quite rapidly. If the increase in temperature rises to apoint that is not necessarily desired, then portions of the foil couldliterally melt and perhaps cause the adhesive outer layers to bum insome circumstances. The fusible links (portions) involve the areas at834, such that if any melting is going to occur in the foil layers, itwill likely occur in the areas that have the highest current density.This will be the areas at 834, and the current densities will bemaximized within the short distances at 836 and 838.

The area created by the short distances 836 and 838 will observe themaximum current density and thus will melt first. When this meltingoccurs, the induction heating tool will observe a brief time intervalwhere its input power readings will cause it to believe that it hadprovided a maximum, or more than sufficient, energy such that it shouldnow reduce its duty cycle. After a short time interval, the eddycurrents in the susceptor 830 will again settle down, as will theback-EMF, and the induction heating tool will observe an input powerreading that allows it to again decide whether or not to increase powerto a point that is sufficient to continue to raise the temperature ofthe foil layer of the susceptor 830. In the meantime, the foiltemperature will briefly have been allowed to cool, and therefore, theadhesive outer layers will not tend to bum.

It will be understood that the susceptor 830 illustrated in FIG. 20 willnot exhibit “even heating” when exposed to a magnetic field; in fact,quite the opposite will occur. This is in contrast to the Boeingmethodologies of bonding, by which “even heating” is always an importantcriterion, both for structural strength and for air-tightness orliquid-tightness in particular portions of an aircraft. In the presentinvention, the susceptor 803 is more useful for bonding to buildingwalls, floors, ceilings, etc. (denoted as “substrates,” as discussedbelow in reference to FIG. 22), and the very even-heating aspect ofbonding is not always required, so long as the necessary overallstrength is achieved. The present invention is quite useful in creatingbonds of such necessary overall strength, and “even heating” is usuallynot a requirement.

FIG. 21 illustrates another susceptor 840 that not only includes fusiblelinks or portions, but also has a pattern of holes or cut-outs that willtend to create a more even heating distribution, by tending to cause thecurrent density to be more equalized throughout the inner areas of thesusceptor 840. Susceptor 840 includes two outer rows of holes orcut-outs at 842. In addition, there are three “new” rows of such holesor cut-outs at 844, which run down the longitudinal axis of thesusceptor 840. There are fusible areas at 846 and 848 that will tend tocontrol the current flow of the foil portion of the susceptor 840.

As described in reference to FIG. 20, if the current density increasestoo rapidly, then the areas of the foil having the highest currentdensity will tend to melt and create an open circuit in those very sameareas. When that occurs, the current density will observe a suddenchange in certain areas of the susceptor 840, and moreover, anotheralternative embodiment of the present induction heating tool willobserve a change in an attribute that can cause the magnetic field beingoutput by the heating tool to temporarily reduce in magnitude. Thisprevents the outer adhesive layers from burning, which will result in abetter bond of the adhesive materials.

Referring now to FIG. 22, a single-foil susceptor is illustrated, afterit has been used to bond two different substrate structures together.The susceptor itself comprises the foil layer 860 and two adhesivecoatings 862 and 864. It will be understood that FIG. 22 is not toscale.

After the heating has occurred, the adhesive surfaces 862 and 864 arebonded to the substrates 852 and 854, respectively. As discussed above,it is preferred that a thin foil susceptor be used, in which thethickness of the susceptor (at 870) is kept to a maximum of two (2) mils(51 microns), or perhaps as much as three (3) mils (76 microns) incertain applications. In many circumstances, the preferred thickness 870of the susceptor foil layer 860 will be as small as (or even smallerthan) one-half mil (13 microns).

Proper activation of the adhesive coating of the susceptor requires thatthe adhesive (at 862 and 864) reach a specified temperature, be heldthere for a minimum time, then cool down and “set” as quickly aspossible. Inductive heating of a thin metallic susceptor results inenergy deposition in the susceptor 860, and this energy conducts intothe substrates 852, 854 and the adhesive 862, 864. The temperaturereached depends on the power level of the deposition, the thermalconduction into the substrate and adhesive, and the duration of heating,and the power delivery profile.

If the deposition power level is high, then the specified temperaturewill be reached in a short time. Only a small portion of the energy willbe conducted into the substrates 852, 854 and, if the power is turnedoff after only a very short energization time interval, the susceptor860 and adhesives 862, 864 will cool down quickly due to continuedconduction into the (mostly still cool) substrates 852, 854. If thepower level is too high and for a very short application time interval,and the energy deposition is thus too low, the adhesives 862, 864 willbe inadequately activated and a poor bond will result.

If the deposition power level is low, then the specified temperaturewill be reached only after a relatively long application time. A largeamount of energy will be conducted into the substrates 852, 854 andlarge regions of the substrates proximal to the susceptor 860 will beheated. When the deposition power is turned off, the susceptor 860 andactivated adhesives 862, 864 will fall slowly in temperature due to thislarge amount of energy now stored in the substrates 852, 854. So long asthe application time interval is sufficiently long, an adequate bondwill be formed between the susceptor 860 and substrates 852, 854.

However, if the deposition power level is too high, and the cool-downtime too high, then the adhesives 862, 864 will not “set” quickly, andthe work pieces (i.e., the substrates 852, 854) will have to be held inplace long after the heating cycle has been completed. This is not adesired outcome since assembly time increases, and further the chance ofhuman error increases that could result in poor assembly of the workpieces.

The heat transfer rate due to the thermal diffusion coefficients fromthe foil to adhesive and from the adhesive to the substrates, is veryimportant. If the thermal diffusion coefficients are too large, then thesubstrates will not be sufficiently raised in temperature before thesusceptor's foil (860) would melt, or the adhesives (862, 864) would bumor crystallize. On the other hand, if the application time interval istoo long, then the substrates (852, 854) will become too hot for theadhesive to quickly set, as noted above.

There is an optimum power level and duration which adequately activatesthe adhesives 862, 864 while not incurring a long clamping-time penaltyduring cool down. This optimum depends on the thermal activity and heatcapacity of the substrate 860, and in the activation characteristics ofthe adhesives 862, 864. It also, of course, depends upon the power levelof the magnetic field being generated by the work coil of the inductionheating tool 10, the distance between the work coil and the susceptor860, and the application time interval during which the magnetic fieldis produced.

In the present invention, the most useful application time intervalsrange from 0.2 seconds to over ten (10) seconds. Of course, for quickassembly of work pieces, the shorter application times are desired. Toachieve the desired quick-assembly times, a large amount of magneticenergy produced by the work coil is necessary, which thus producesrelatively large amounts of thermal energy in the susceptor due to eddycurrents. For thin foil susceptors, a maximum power of about two (2) kWper square inch of foil can be used as the average power over the entireheating event (i.e., during the application time interval). Thesusceptor 860 must be properly designed to handle this much power, sincetoo much power for too long a time interval will certainly melt portionsof the susceptor; in fact, too much power could literally make thesusceptor explode.

The induction heating tool of the present invention has the capabilityof operating very effectively at distances of at least 0.75 inches (19mm) between the work coil and the susceptor 860. At this range ofdistances, the application time for producing the magnetic field can beas little as 0.05 seconds. The present induction heating tool 10 canalso be used at much greater distances if the application time intervalis increased, including a distance of at least three (3) inches (76 mm).The preferred range of application time intervals can run from 0.05second through 10 seconds, or more preferred from 0.10 seconds through 5seconds, or most preferred from 0.15 seconds through 2 seconds.

FIG. 23 diagrammatically illustrates a person utilizing the inductiontool, which can be broken out into more than one portion. In FIG. 23,the induction heating tool is generally depicted by the referencenumeral 900, and has a battery pack at 902 that is worn around the waistof the human user on a belt 904. A power cable 906 provides theelectrical power from the battery pack 902 to the induction heating tool900. FIG. 24 shows a similar arrangement, however, the induction heatingtool 910 is used with a battery pack 912 that is worn on a shoulderharness 914 that is slung cross-ways from the right shoulder to the lefthip. A power cable 916 carries the battery power from the battery pack912 to the induction heating tool 910.

FIG. 25 again shows an induction heating tool 920 with a battery pack;in this case the battery pack 922 is worn on a backpack that is attachedto a pair of shoulder harnesses 924. A battery cable 926 is providedbetween the battery pack 922 and the heating tool 920.

Another example use of the induction heating tool is depicted in FIG.26, in which the heating tool 930 can be utilized with either a batterypack or with AC line voltage. The battery pack at 932 is illustrated asa bandoleer-type belt, having a battery cable 938. Alternatively, apower cable 936 and a plug-in battery charger or AC adapter 934 areillustrated. Individual batteries 940 can be replaced on the bandoleerconstruction at 932.

It will be understood that the logical operations described in relationto the flow charts of FIGS. 17A-17D can be implemented using sequentiallogic, such as by using microprocessor technology or using a logic statemachine; it even could be implemented using parallel logic. Thepreferred embodiments use a microprocessor to execute softwareinstructions that are stored in memory cells. In fact, the entiremicroprocessor (or microcontroller) and certain memory cells could becontained within an ASIC, if desired. Of course, other circuitry couldbe used to implement these logical operations, without departing fromthe principles of the present invention, such as the analog circuitdescribed in FIG. 16.

It will be further understood that the precise logical operationsdepicted in the flow charts of FIGS. 17A-17D and discussed above, couldbe somewhat modified to perform similar, although not exact, functionswithout departing from the principles of the present invention. Theexact nature of some of the decision steps and other commands in theseflow charts are directed toward specific embodiments of inductionheating tools, and certainly similar, but somewhat different, stepswould be taken for use with other sizes or shapes of heating systems inmany instances, although the overall inventive results would be thesame.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Obvious modifications or variations are possible in light ofthe above teachings. The embodiment was chosen and described in order tobest illustrate the principles of the invention and its practicalapplication to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. It isintended that the scope of the invention be defined by the claimsappended hereto.

What is claimed is:
 1. A heating apparatus, comprising: a work coil andan electronic circuit, said electronic circuit, upon a actuation of acycle, being configured to determine both a current magnitude and avoltage magnitude input to a power converter stage over a plurality ofsampled time intervals, then being configured to average said currentand voltage magnitudes to calculate a power level being consumed by thepower converter stage; and said electronic circuit being furtherconfigured to adjust said power level according to a predeterminedenergy delivery scheme until achieving a predetermined accumulatedenergy for said predetermined energy delivery scheme, and terminatingsaid cycle.
 2. The heating apparatus as recited in claim 1, wherein saidpredetermined energy delivery scheme comprises a profiled energydelivery scheme.
 3. The heating apparatus as recited in claim 1 whereinsaid power converter stage comprises an inverter circuit.
 4. The heatingapparatus as recited in claim 1, wherein one of a continuous operatingmode or a one-shot operating mode is selectable by a user; and whereinthe continuous operating mode repeatedly self-triggers a subsequentenergy delivery cycle for so long as said user maintains said actuation,while the one-shot operating mode requires a separate user actuation foreach subsequent energy delivery cycle.
 5. The heating apparatus asrecited in claim 4, wherein said power level must fall between alow-load limit and an overload limit; otherwise if the power level isless than said low-load limit or greater than said overload limit, thenin said one-shot operating mode said heating apparatus operation will beimmediately halted.
 6. The heating apparatus as recited in claim 4,wherein said power level must fall between a low-load limit and anoverload limit; otherwise if the power level is less than said low-loadlimit or greater than said overload limit, then in said continuousoperating mode said heating apparatus operation will continue for apredetermined number of subsequent said sampled time intervals at aminimum power level condition, until (a) said power level achieves avalue between said low-load and overload limits, then continuing normaloperation, or (b) if said power level fails to achieve a value betweensaid low-load and overload limits, then halting said heating apparatusoperation.
 7. The heating apparatus as recited in claim 2, wherein saidelectronic circuit is further configured to determine whether apredetermined maximum time interval has occurred before said profiledenergy delivery scheme has been achieved, and if so declaring a timeouterror and halting said heating apparatus operation.
 8. The heatingapparatus as recited in claim 2, wherein said profiled energy deliveryscheme comprises a plurality of successive time intervals during whichsaid electronic circuit is configured to control said power level duringeach said time interval, and is further configured to adjust said powerlevel in subsequent said time intervals until reaching saidpredetermined accumulated energy.
 9. The heating apparatus as recited inclaim 1, wherein the current and voltage magnitudes are determinedduring a portion of said cycle, and their average magnitudes are used tocalculate said power level during said portion of the cycle; and saidpower level is adjusted for at least one subsequent portion of thecycle.
 10. The heating apparatus as recited in claim 4, wherein anelectrical current level must fall between a low-load limit and anoverload limit; otherwise if the current level is less than saidlow-load limit or greater than said overload limit, then in saidone-shot operating mode said heating apparatus operation will beimmediately halted.
 11. The heating apparatus as recited in claim 4,wherein an electrical current level must fall between a low-load limitand an overload limit; otherwise if the current level is less than saidlow-load limit or greater than said overload limit, then in saidcontinuous operating mode said heating apparatus operation will continuefor a predetermined number of subsequent said sampled time intervals ata minimum current level condition, until (a) said current level achievesa value between said low-load and overload limits, then continuingnormal operation, or (b) if said current level fails to achieve a valuebetween said low-load and overload limits, then halting said heatingapparatus operation.
 12. A heating apparatus, comprising: a work coiland an electronic circuit, said electronic circuit, upon an actuation ofa cycle, being configured to determine both a current magnitude and avoltage magnitude input to a power converter stage over a plurality ofsampled time intervals, then being configured to average said currentand voltage magnitudes to calculate a power level being consumed by thepower converter stage; and said electronic circuit being furtherconfigured to adjust said power level according to a profiled energydelivery scheme for delivering predetermined amounts of energy.
 13. Theheating apparatus as recited in claim 12, wherein said predeterminedamounts of energy are delivered to a susceptor.
 14. The heatingapparatus as recited in claim 12, wherein said power level is adjusteduntil achieving a predetermined accumulated energy for said profiledenergy delivery scheme, then said cycle is terminated.
 15. The heatingapparatus as recited in claim 12, wherein said power converter stagecomprises an inverter circuit.
 16. The heating apparatus as recited inclaim 12, wherein one of a continuous operating mode or a one-shotoperating mode is selectable by a user; and wherein the continuousoperating mode repeatedly self-triggers a subsequent energy deliverycycle for so long as said user maintains said actuation while theone-shot operating mode requires a separate user actuation for eachsubsequent energy delivery cycle.
 17. The heating apparatus as recitedin claim 16, wherein said power level must fall between a low-load limitand an overload limit; otherwise if the power level is less than saidlow-load limit or greater than said overload limit, then in saidone-shot operating mode said heating apparatus operation will beimmediately halted.
 18. The heating apparatus as recited in claim 16,wherein said power level must fall between a low-load limit and anoverload limit; otherwise if the power level is less than said low-loadlimit or greater than said overload limit, then in said continuousoperating mode said heating apparatus operation will continue for apredetermined number of subsequent said sampled time intervals at aminimum power level condition, until (a) said power level achieves avalue between said low-load and overload limits, then continuing normaloperation, or (b) if said power level fails to achieve a value betweensaid low-load and overload limits, then halting said heating apparatusoperation.
 19. The heating apparatus as recited in claim 12, whereinsaid electronic circuit is further configured to determine whether apredetermined maximum time interval has occurred before said profiledenergy delivery scheme has been achieved, and if so declaring a timeouterror and halting said heating apparatus operation.
 20. The heatingapparatus as recited in claim 12, wherein said profiled energy deliveryscheme comprises a plurality of successive time intervals during whichsaid electronic circuit is configured to control said power level duringeach said time interval, and is further configured to adjust said powerlevel in subsequent said time intervals until reaching saidpredetermined accumulated energy.
 21. The heating apparatus as recitedin claim 12, wherein the current and voltage magnitudes are determinedduring a portion of said cycle, and their average magnitudes are used tocalculate said power level during said portion of the cycle; and saidpower level is adjusted for at least one subsequent portion of thecycle.
 22. The heating apparatus as recited in claim 16, wherein anelectrical current level must fall between a low-load limit and anoverload limit; otherwise if the current level is less than saidlow-load limit or greater than said overload limit, then in saidone-shot operating mode said heating apparatus operation will beimmediately halted.
 23. The heating apparatus as recited in claim 16,wherein an electrical current level must fall between a low-load limitand an overload limit; otherwise if the current level is less than saidlow-load limit or greater than said overload limit, then in saidcontinuous operating mode said heating apparatus operation will continuefor a predetermined number of subsequent said sampled time intervals ata minimum current level condition, until (a) said current level achievesa value between said low-load and overload limits, then continuingnormal operation, or (b) if said current level fails to achieve a valuebetween said low-load and overload limits, then halting said heatingapparatus operation.
 24. A heating apparatus, comprising: a work coiland an electronic circuit, said electronic circuit, upon actuation of acycle, being configured to determine both a current magnitude and avoltage magnitude input to a power converter stage over a plurality ofsampled time intervals, then being configured to average said currentand voltage magnitudes to calculate a power level being consumed by thepower converter stage; and said electronic circuit being furtherconfigured to adjust said power level according to a profiled energydelivery scheme until achieving a predetermined accumulated energy forsaid profiled energy delivery scheme, and terminating said cycle. 25.The heating apparatus as recited in claim 24, wherein said powerconverter stage comprises an inverter circuit.
 26. The heating apparatusas recited in claim 24, wherein one of a continuous operating mode or aone-shot operating mode is selectable by a user; and wherein thecontinuous operating mode repeatedly self-triggers a subsequent energydelivery cycle for so long as said user maintains the original actuationof the heating apparatus, while the one-shot operating mode requires aseparate user actuation for each subsequent energy delivery cycle. 27.The heating apparatus as recited in claim 26, wherein said power levelmust fall between a low-load limit and an overload limit; otherwise ifthe power level is less than said low-load limit or greater than saidoverload limit, then in said one-shot operating mode said heatingapparatus operation will be immediately halted.
 28. The heatingapparatus as recited in claim 26, wherein said power level must fallbetween a low-load limit and an overload limit; otherwise if the powerlevel is less than said low-load limit or greater than said overloadlimit, then in said continuous operating mode said heating apparatusoperation will continue for a predetermined number of subsequent saidsampled time intervals at a minimum power level condition, until (a)said power level achieves a value between said low-load and overloadlimits, then continuing normal operation, or (b) if said power levelfails to achieve a value between said low-load and overload limits, thenhalting said heating apparatus operation.
 29. The heating apparatus asrecited in claim 24, wherein said electronic circuit is furtherconfigured to determine whether a predetermined maximum time intervalhas occurred before said profiled energy delivery scheme has beenachieved, and if so declaring a timeout error and halting said heatingapparatus operation.
 30. The heating apparatus as recited in claim 24,wherein said profiled energy delivery scheme comprises a plurality ofsuccessive time intervals during which said electronic circuit isconfigured to control said power level during each said time interval,and is further configured to adjust said power level in subsequent saidtime intervals until reaching said predetermined accumulated energy. 31.The heating apparatus as recited in claim 24, wherein the current andvoltage magnitudes are determined during a portion of said cycle, andtheir average magnitudes are used to calculate said power level duringsaid portion of the cycle; and said power level is adjusted for at leastone subsequent portion of the cycle.